Here is the output when I connected my computer to the internet source using an ethernet cable. From the enp0s25 interface you can see it is acquiring IP via DHCP.
and add new interface 3g in wan firewall zone.
I would also advise to reset the router to defaults, after you take a backup. wan and lan will work immediately and you'll only need to apply the configuration for 3G. This way you'll make sure that no other wrong configuration is still in the router.
After the reset the router will work without any additional setting. The lan will have 192.168.1.1/24 and wan will acquire some IP from the 192.168.0.0/24 subnet.
After you make sure it works, you can add the configuration for the 3g.
So we backedup, reset and configured and still it behaves the same way.
We noticed that if you switch WAN ifname to eth0.1 and LAN ifname to eth0.2, the WAN starts working but the LAN stops and if you switch it back, the LAN starts working and WAN stops.
You can temporarily allow SSH from the WAN interface.
In addition, you can create an alias interface that will have a private static IP different from your LAN and WAN subnets.
I created an Alias interface with an IP address of 10.0.0.1. when I connect an internet source to the WAN port of the router, it stops working ( i.e I will not be able to ssh into the Alias interface I created) but once I disconnect the internet source from the WAB port of the router, it starts working ( i.e I will be able to ssh into the Alias interface)
In the logs I can see port 4 going up/down. Fri Sep 25 15:03:36 2020 kern.info kernel: [ 730.547827] mtk_soc_eth 10100000.ethernet eth0: port 4 link down
This port however belongs to lan.
So my question is: are you plugging the cable to the right port?
Then there is something wrong with the mapping of the ports and you need to identify yourself which one is the wan port.
Open Network/Switch in Luci web page and you'll see there the port that is connected to your PC. Connect some other pc to the remaining ports and see on the top which port is marked as connected. Do that until the wan port (the one which is marked as untagged on vlan2) shows as connected.