Obtained a "new" ZyXEL VMG3925-B10B modem/router from e-bay at a very good price, seller guaranteed it was not an ex ISP model, on checking this certainly appeared to be the case.
Initial GUI access with admin/1234, PW requested changing to min. 8 mixed characters, accepted.
Followed this very informative post for detailed guidance:
https://forum.kitz.co.uk/index.php/topic,20006.msg350991.html#msg350991The unit was running V5.13(AAVF.10)C0 firmware which was not showing a BusyBox prompt on Telnet access.
Loaded V5.13(AAVF.7)C0 FW which did permit BusyBox access, successfully extracted and cracked the supervisor PW..
Now decided to check the latest available FW V5.13(AAVF.13)C0, found this had the later ZyXEL GUI interface which I had experienced before on my XyXEL VMG1312-B10D, not to my liking, also did not permit BusyBox access.
Experimented with FW revisions and established that V5.13(AAVF.9)B0 was the latest rev. that permitted BusyBox access and still had the original ZyXEL GUI which I prefer.
Note: The admin & supervisor PW's remained the same during all the FW changes.
Also noted that FW changes from newer to older revisions usually had to be done in sequence, ie. moving back by more than one step produced a "not valid" file error.
Last move was migration from FW V5.13(AAVF.10)B0 to V5.13(AAVF.9)B0, this appeared to proceed normally but now found I could not access the GUI at any level with any password !
Tried the push button reset procedure, power down/re-boot etc. but no joy, locked out.
Obviously can't get BusyBox access either without a PW..
So, do I now have to go down the serial interface route ?
If so will clearing "ROM D" restore default admin access ?
Is the FW corrupted and if so how can I reload ?