This is getting a little confusing.
The default configuration, as when installed by Openreach, is the correct configuration. The simplest way of reverting the HG612 back to that configuration (whilst still retaining the unlocked status) is to press and hold the reset button for at least 10 seconds. (10, 20 or 30 seconds will all effectively have the same result . . . a "long reset"). Once you have released the reset button do not attempt to access the device, power-cycle it, etc, for the next five minutes. Just leave it alone.
Once that five minutes has elapsed, you could power-cycle the device. It will then be configured as a pure bridge, using a VLAN tagged 101.