Just to complement the previous advice: remember to also set the gateway and DNS address on the new dumb AP. Otherwise the router itself will be unable to access internet (although clients connected via it will be, as they use DHCP to get address from the main router.)
Old advice written by me:
If you connect the two routers using a fixed connection between LAN ports, the whole thing is ridiculously easy, as you can dumb down the APs. (The key to making things really easy is to NOT use the WAN port in the slave APs at all.)
Basically you need to adjust these in router2:
- set the hostname to be different than the main router has
- set router2’s LAN interface to match the subnet of router1, but to have a different fixed IP address. I use 192.168.1.2
- set that router’s DNS and gateway info to point into router1 (192.168.1.1 in my case)
- config the DHCP server in router2 to ignore requests from local LAN interface. I do that config to prevent mishaps with the service starting accidentally, although I also disable the service itself (dnsmasq).
- disable the DHCP/DNS service (dnsmasq) in router2
- config the Wifi normally. At least change the wifi channel to prevent signal overlap.