Telnet - CLI - Add web configurator password without GUI

I have a Zyxel Router (VMG3925-B10B) which I am trying to configure to replace my BT Home Hub 5 (which might I say, is awful, and horrible, in every possible way). The problem is, I cannot get logged in, none of the typical and previously working credentials are working - I just cant get logged in.

I tried something different, Telnet onto the IP and got logged in and it works, I am on the router, and I can see some information (I managed to get the Firmware version for example).

I am wondering if anyone is able to help me, I am wanting to use the CLI through Telnet, to create a new user credential so I can get logged into the routers page. Primarily I want to reload the firmware on the box, its running an old version so I want to try the new firmware, and if that fails - rollover to an older release.

I am currently awaiting an TTL USB and cable set as I might have to reload the firmware through cable, but would much rather find a way to get around the problem without going down the line of risking turning it into a brick...

Anyone able to help?

#CPE_May