Okay, since these were not mentioned...
Have you tried:
admin/[blank]
admin/admin
admin/<serial number>
admin/service
It sounds too easy, but sometimes the most hardest of things, require an easy solution.
Also, you might want to check your AUP - if the provider finds out you're tampering with the MDC and the modem tells them that, you're going to have a lot of explaining to do.
Also it would help us all if you included:
Modem manuf, type, provider
Good luck.