This is my set up.

I have a 3G router with aircard ( USB modem?) connected to it.

The house is completely network wired, and I have LAN hub to which my PC is connected and all the rooms, as well as the router.

In the settings, and also Mr Google tells me, that if I use a USB modem I should connect through the WAN port.

However if I do that I can't connect to the router from my PC, nor can the devices in other rooms connect to the internet. If I plug the cable into a LAN port of the router everything is working fine.

What am I missing?

LAN ports are for Local equipment and devices

WAN ports are for upstream connection to other networks (including the Internet)

Your Router probably actually has 'two' WAN ports, one physical Ethernet Port for connecting to physical uptream ISP equipment (ie DSL or Fiber modem) and a second 'soft' port that used the USB physical connection to alternate Internet connectivity (such as a 3G/4G/LTE 'air' interface).

So, the USB modem 'is' connecting through the WAN port, the rest of your local equipment and devices connect through the LAN or WLAN port. This is necessary for the router to offer Internet Access to multiple devices by sharing a single WAN IP Address utilizing NAT (Network Address Translation).