Can't change Device Access Code on Netgear B90-755025-15 router

Sounds to me like the code has already been changed to prevent tampering. A gateway reset would be necessary to use the printed code to get into the Netgear menu and make a change. A reset would wipe out all your connection settings. Do you really want to do that?

What would I have to do to do that?

I beleive the reset button is on the side of the gateway with a red circle around it. I have never seen instructions/manual for the 7550 but typical routers use a paper clip or similar item to reset. Do a search on lenght of time (usually 30 secs) and and off/on cycle.

I've done that a few times before, and it didn't work.

Did it shutdown and reboot?

You could be working with a brick.

Yes, it did restart.

Sounds like you're S0L.

Could the code have been swapped with another device and would ATT keep track of serial nos and codes?

Call them and ask is my last suggestion.