@Barxclan 

Hey

So yeah you sound like you have solved this yourself.   The AX20 router is a gateway router, namely its designed to be connected to a modem or ISP router to provide internet access, connect this via the port in the back labelled WAN..  Its usually blue in colour

In terms of a modem for this, would you have your old ISP router?   This would work as you can disable the WiFi on it, then connect the WAN port on the AX20 to any of the LAN ports on the ISP Router.    Basically in that setup you would have the old ISP router doing modem only.. everything will be controlled by your new AX20

If you don't have your ISP router, get a cheap WAN router or modem from someone and that should work too.   Just the most basic DSL modem or router will work unless you have super fast internet.. honestly cheap as you get its doing nothing :)

In terms of the mesh setup, that will work also but if you have a large house with thick walls its going to be trial and error if honest!