I'm not sure why you'd get a conflict with the WAN address. You didn't say which router, but your internal LAN IPs should not be on the same subnet as the WAN IPs in the first place.

ISP >> Modem >> WAN port of Main router

Main router LAN port >> Second router LAN port (do NOT use WAN port on the second router)

You didn't give any make/model of the routers, so I can only guess.
If anything, you could use "Access Point" mode on the second router. Router is "normal" router mode. Bridge would be to link 2 routers together as a dedicated bridge (no clients).