I don't understand what you are trying to accomplish. In my situation the customer is using the Cable company's modem, connected to an edgerouter X which is the router but doesn't have an wifi capability. Instead of getting him a unifi AP to provide wifi, I just used his previous router, the TP-Link Archer, turned off the routing features and only used the wifi part, thus turning it into an access point.
I only use bridge mode on the ISP-supplied gateway combos to use my own router instead of the built in one from the ISP's combo. In our case, it wasn't a gateway combo, just a plain modem.