Changed my modem VMG1312-B10A tonight with a spare to rule everything out. Strange thing is the modem had forgotten the admin password and couldn't log in. So I telnet in with Supervisor password and ran dumpmdm to a file and it was in the file the correct password for admin but still wouldn't login just got incorrect username or password.
So I logged into the gui with Supervisor password which worked and went to users and deleted the admin user, then added the admin user back with a firefox generated password and now I can login as admin. Very strange it has just been sitting on a shelf with the other backup.
AC68U is due tomorrow.