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Whenever I add my 4G modem to my raspberry, it gets on top of the default routes ou ip route list, however I want everything to go through wlan, and only use the 4G modem to receive SSH connections.

I've found this answer on how to disable the default routes.

however, after reboot, the 4G modem comes back to the top.

How do I make wlan0 to always be the first rule on default?

UPDATE:

Here's the dmesg output when I connect the USB dongle:

[426102.910168] usb 1-1.5.1: new full-speed USB device number 6 using dwc_otg

[426103.046670] usb 1-1.5.1: not running at top speed; connect to a high speed hub

[426103.056674] usb 1-1.5.1: New USB device found, idVendor=12d1, idProduct=1f01

[426103.056693] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3

[426103.056704] usb 1-1.5.1: Product: HUAWEI_MOBILE

[426103.056714] usb 1-1.5.1: Manufacturer: HUAWEI_MOBILE

[426103.056724] usb 1-1.5.1: SerialNumber: 0123456789ABCDEF

[426103.121355] usb-storage 1-1.5.1:1.0: USB Mass Storage device detected

[426103.122875] scsi host0: usb-storage 1-1.5.1:1.0

[426103.987177] usb 1-1.5.1: USB disconnect, device number 6

[426105.470211] usb 1-1.5.1: new full-speed USB device number 7 using dwc_otg

[426105.606666] usb 1-1.5.1: not running at top speed; connect to a high speed hub

[426105.615673] usb 1-1.5.1: New USB device found, idVendor=12d1, idProduct=14dc

[426105.615692] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0

[426105.615703] usb 1-1.5.1: Product: HUAWEI_MOBILE

[426105.615713] usb 1-1.5.1: Manufacturer: HUAWEI_MOBILE

[426105.766297] usb-storage 1-1.5.1:1.2: USB Mass Storage device detected

[426105.766768] scsi host0: usb-storage 1-1.5.1:1.2

[426105.855053] cdc_ether 1-1.5.1:1.0 eth1: register 'cdc_ether' at usb-3f980000.usb-1.5.1, CDC Ethernet Device, 0c:5b:8f:27:9a:64

[426105.855593] usbcore: registered new interface driver cdc_ether

[426106.785653] scsi 0:0:0:0: Direct-Access     HUAWEI   TF CARD Storage  2.31 PQ: 0 ANSI: 2

[426106.803758] sd 0:0:0:0: Attached scsi generic sg0 type 0

[426106.820687] sd 0:0:0:0: [sda] Attached SCSI removable disk

Here's ip addr

eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000

    link/ether 0c:5b:8f:27:9a:64 brd ff:ff:ff:ff:ff:ff

    inet6 fe80::584f:751f:bb3e:e26b/64 scope link 

       valid_lft forever preferred_lft forever

UPDATE 2

I attached it a few more times until it showed the eth1 route:

[10787.229141] usb 1-1.5: new full-speed USB device number 7 using dwc_otg

[10787.363515] usb 1-1.5: New USB device found, idVendor=05e3, idProduct=0606

[10787.363533] usb 1-1.5: New USB device strings: Mfr=1, Product=2, SerialNumber=0

[10787.363544] usb 1-1.5: Product: USB Hub 2.0

[10787.363555] usb 1-1.5: Manufacturer: ALCOR

[10787.365166] hub 1-1.5:1.0: USB hub found

[10787.369831] hub 1-1.5:1.0: 4 ports detected

[10797.419094] usb 1-1.5.1: new full-speed USB device number 8 using dwc_otg

[10797.555636] usb 1-1.5.1: not running at top speed; connect to a high speed hub

[10797.565759] usb 1-1.5.1: New USB device found, idVendor=12d1, idProduct=1f01

[10797.565777] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3

[10797.565789] usb 1-1.5.1: Product: HUAWEI_MOBILE

[10797.565799] usb 1-1.5.1: Manufacturer: HUAWEI_MOBILE

[10797.565808] usb 1-1.5.1: SerialNumber: 0123456789ABCDEF

[10797.630477] usb-storage 1-1.5.1:1.0: USB Mass Storage device detected

[10797.631101] scsi host0: usb-storage 1-1.5.1:1.0

[10798.472745] usb 1-1.5.1: USB disconnect, device number 8

[10799.469081] usb 1-1.5.1: new full-speed USB device number 9 using dwc_otg

[10799.630768] usb 1-1.5.1: not running at top speed; connect to a high speed hub

[10799.646891] usb 1-1.5.1: New USB device found, idVendor=12d1, idProduct=14dc

[10799.646909] usb 1-1.5.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0

[10799.646920] usb 1-1.5.1: Product: HUAWEI_MOBILE

[10799.646930] usb 1-1.5.1: Manufacturer: HUAWEI_MOBILE

[10799.814489] usb-storage 1-1.5.1:1.2: USB Mass Storage device detected

[10799.815008] scsi host0: usb-storage 1-1.5.1:1.2

[10799.897788] cdc_ether 1-1.5.1:1.0 eth1: register 'cdc_ether' at usb-3f980000.usb-1.5.1, CDC Ethernet Device, 0c:5b:8f:27:9a:64

[10799.898127] usbcore: registered new interface driver cdc_ether

[10800.889652] scsi 0:0:0:0: Direct-Access     HUAWEI   TF CARD Storage  2.31 PQ: 0 ANSI: 2

[10800.910585] sd 0:0:0:0: Attached scsi generic sg0 type 0

[10800.923297] sd 0:0:0:0: [sda] Attached SCSI removable disk

Here's route -n

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 wlan0

0.0.0.0         192.168.8.1     0.0.0.0         UG    207    0        0 eth1

0.0.0.0         192.168.1.1     0.0.0.0         UG    303    0        0 wlan0

169.254.0.0     0.0.0.0         255.255.0.0     U     202    0        0 eth0

169.254.0.0     0.0.0.0         255.255.0.0     U     204    0        0 docker0

169.254.0.0     0.0.0.0         255.255.0.0     U     206    0        0 veth4557ad2

172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 docker0

192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 wlan0

192.168.1.0     0.0.0.0         255.255.255.0   U     303    0        0 wlan0

192.168.8.0     0.0.0.0         255.255.255.0   U     207    0        0 eth1

See that I did ifmetric wlan0 in order to be able to use the wlan0 to ssh into my raspberry

UPDATE 09/10:

allow-hotplug wlan0

iface wlan0 inet dhcp

    up ifmetric wlan0 0

    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

This won't make my wlan0 have metric 0. What am I doing wrong?

For changing the routing priority for an interface you change metrics.

By default, all are 0, which is the highest priority. So, you can do:

allow-hotplug eth1

iface eth1 inet dhcp

    up ifmetric eth1 30

To use ifmetric in Debian, you have got to install it:

sudo apt-get install ifmetric

ifmetric

Set routing metrics for a network interface

ifmetric is a Linux tool for setting the metrics of all IPv4 routes
attached to a given network interface at once. This may be used to
change the priority of routing IPv4 traffic over the interface. Lower
metrics correlate with higher priorities.

The metric 0 means the highest priority route and is the default one.
The larger metric value means lower priority routes. The IP address of
the active interface with the lowest metric value becomes the
originating one. See ifmetric(8).

This is the good old problem of how to override a default route.

The easiest way for IPv4 is not to try and change the metric/priority but split the route:

ip route add 0.0.0.0/1 via wlan0-gw

ip route add 128.0.0.0/1 via wlan0-gw

The reason this works is that routing table lookups are performed using a longest-matching-prefix search. Route metrics come into play only when there are two otherwise equivalent routes. In this case, these two routes have a 1-bit prefix which is longer than the 0-bits prefix of the default route (0.0.0.0/0) and will be consulted first. Since these two routes cover the whole of 0.0.0.0/0, they will always have preference over the default route.

An alternative approach is to use a separate routing table with the default route you prefer and add a rule to send all traffic there. I've never seen any automation that messes with ip rules, so it won't matter what other routes they add. The problem here is that your automation won't be able to insert additional routes though.

So, tl;dr:

iface wlan0 inet manual

    up ip route add 0.0.0.0/1 via wlan0-gw

    up ip route add 128.0.0.0/1 via wlan0-gw

    ....

Or (but you will also have to add all other routes to "table 5"):

iface wlan0 inet manual

    up ip rule add to 0.0.0.0/0 table 5

    up ip route add default via wlan0-gw table 5

    ....

One way to Persist the ip route add/del in debian is to write them down into /etc/network/interfaces like below:

allow-hotplug eth1

iface eth1 inet dhcp

    post-up ip route del default

    post-up ip route add default via <gateway IP> dev wlan0

I think the matter on default gateway. Here is the tutorial.

I briefly explain the steps. First, you need to delete the default gateway and add the one you want to add. Then, you edit "/etc/network/interfaces" for permanent change. Finally, you restart the service.

https://www.wikihow.com/Add-or-Change-the-Default-Gateway-in-Linux