Nearly all ASUS routers built in the last 2-3 years, are compatible with their AiMesh system. A few of the oldest in this range just need the latest firmware update to support it.
The RT-AX1800S is AiMesh capable (its right their in the management interface). I just got one to add to my RT-AX82U.
To set it up, you essentially set up the node as a regular wireless router plugged into a LAN port on what will be the master, then enable AiMesh in the node, and reboot, then go to the master, and add an AiMesh node, and it will list the available AiMesh nodes, pick the RT-AX1800S from the list of avallable nodes, and bingo.
I have not done it yet, because of the difficulty getting an ethernet cable from a LAN port on the RT-AX82U to the WAN port on the RT-AX1800S due to where they are positioned in the house, so that I can have a wired-backhaul on the RT-AX1800S as a mesh node with the RT-AX82U as the master - wireless backhaul introduces double the hops and latency, so it is not really an acceptable solution for me.

