[masraum]

Most routers have a "WAN" port and then several "LAN" ports. The WAN side faces the Internet and the LAN side faces the PCs/devices that need Internet access.

Normally, the WAN side gets an IP from the service provider, and then the LAN side gets IPs from the router (192.168.0.x, or 192.168.1.x or something similar is common).

It depends upon the routers, but IME, the best way to connect things would be

router #1 (yours) LAN connection ----> router #2 LAN connection. Then access the config of router #2 and disable DHCP. You don't want router #2 giving out addresses to devices that overlap or conflict with router #1. This normally will just extend your network to the second device. In this config, the second router isn't routing, and doesn't need to see the Internet. It's just extending your network, and all of the tenant devices will be on the same network as your devices.

But that may not work or it may be difficult to get that second router setup for that to work easily. I've done similar in the past and it was pretty easy and worked well, but that depends upon the routers.

A second option would be to run a LAN connection from router #1 to the WAN connection of router #2. Then router #2 will get an address from your router, and all of the devices behind router #2 will get addresses from router #2. There is a small benefit to this method because it will somewhat separate the tenant network from your network. You may not want both of you on the same network in case the tenant wants to try to access your stuff or poke around on your devices. That's still possible, but a bit harder to see.

One note for either/both setup would be to make sure that both routers have different SSIDs. So you could have "Bills-wifi" and "Bills-wifi-extended" (or whatever you want). And it's best if they also use different channels/RF bands, but modern routers may be smart enough to do this automatically.