I have two OpenWRT routers running 25.12, an XR500 and an MR8300. The XR is my main router and the MR serves as the dedicated adblocker for IoT devices, since I didn’t want the adblocker on my main network and didn’t want to mess with a second dnsmasq instance.
But the connection drops often, so I was thinking of just connecting the MR over ethernet since the two routers are close together. Can I plug the MR’s WAN ethernet directly into one of my XR’s physical ports and have everything just work? Would I have to change the MR’s lan subnet to an unused one, like in the wifi client instructions? Would there be anything else that’s needed?
The wifi and physical ports on the XR are separated by VLANs and firewall rules, if that is relevant.
Yes, but wan-side and lan-side subnets mustn't overlap, so you need to change one (or both, it often helps to avoid 192.168.0.0/24, 192.168.1.0/24 and 192.168.178.0/24, which are common defaults) of your routers to a different 192.168.x.0/24 subnet.
Connecting routers/ APs via ethernet is always preferable to a wireless link, massively so. However cascading routers is rarely a sensible choice (unless you're literally dealing with vast campus wide networks with orthogonal responsibilities and requirements or want to simulate those in a lab environment), multiple dnsmasq instances are a small price to pay in comparison. While you can test it this way (because it's quick and easy), I would not recommend to use it this way long term. Generally speaking, there should only be one router in your network, only policy maker - everything else (bridged APs and managed switches) should merely blindly execute the centrally managed policies of your main/ only router