I’m guessing that both routers are running DHCP and/or running NAT at the same time. You need only one to do all that (the one attached to the WAN/Internet) and the other needs to act as a switch only (usually called access point mode).

Not sure about all the mesh stuff, but traditionally you’d switch off all the routing functions of the second router and connect it to the other router via the LAN ports. Again, DHCP and actual routing would need to be turned off on the “secondary” routers.

If all these “routers” (really we should be calling them access points as) are part of the same mesh networking system I’d imagine they’d take care of these issues on their own though, so more details about the network topology would be necessary to be helpful.