I have checked, this is based on Luna 3.3 SDK (quite old solution, still supported, but recommended for these devices is UnifiedSDK 1.x or 2.x - this is what we (leolabs.pl / leox) use.
you can also change this RTK_GW via CLI:
flash get DEVICE_NAME
result - DEVICE_NAME=RTK_GW
command to change is
flash set DEVICE_NAME GP542R
And it will be ok as long as... you do not reset router to defaults.
About user access - try user/user
you can change user's password via CLI:
flash set USER_PASSWORD newtextpassword