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.
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)
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.
So I checked the Network/Switch in Luci web page for a TP-Link MR3420 that runs openwrt and figured out a few things were not done correctly on my current device. The TP-Link works perfectly when cable is connected to WAN.
So i replicated the same configurations. Good news is that the IP address of the LAN no longer changes when you connect the cable to the WAN port but the bad news is that the device is unable to connect to the internet.
So I realized that on TP-Link once you connect cable to the WAN port, the WAN interface on Network/Switch in Luci web page comes up and the CPU interface also comes up but for my current router, only the WAN interface/port 5 comes up and the CPU interface is down.