So you have an ethernet bridge between two wifi routers, with a direct, cabled ethernet connection to your miner to the remote side of the bridge.  And you want your miner to connect to the wifi (far side of the bridge) if the bridge goes down?  What kind of miner is this?  You'd probably have to run a scheduled script or have both connections active at the same time.

My recommendation is to always home run a cable to devices that are critical.  Wifi, while it can be reliable, will always have more interference/trouble than a properly cabled connection.