Just reset the HG612 to its default state.
Press and hold the reset
button for more than ten seconds. Then leave the device alone for at least five minutes
. . . go and make a mug of tea, for example.
After having left the HG612 in a quiescent state, now power cycle it. It will now be as originally configured . . . as a pure modem bridge.
100% Linux and, previously, Unix.