To disable the bridge mode on the modem without hardwired connection or without accessing modem GUI you can reset modem.  At the back of the modem, in the middle section you will see a small hole, inside that hole there is a button, and that is the reset button, please insert a paper clip or a tip of a pen and hold it for 15-30 seconds until all your lights go off, afterwards wait until your device fully boots up. Once the modem rebooted completely you can access Wi-Fi from SBG6580 using default Wi-Fi credentials mentioned on the modem label.