Switching modem from Annex A to Annex B

Greetings.

Here’s the situation: I was operating under the assumption that Annex A would cover my single line for both phone and data, while Annex B would be reserved for ISDN.

I went ahead and ordered an Annex A modem/router combo since I have everything running through one line via AT&T, only to find out while waiting for the delivery that AT&T exclusively uses Annex B, regardless of whether there's an ISDN setup or not.

So, my question is this:
Is there any way to run an Annex A modem/router on an Annex B provider's network? Could I perhaps flash different firmware to make it work, or is there some other workaround?
Or is the difference purely hardware-based, meaning I’ve simply wasted my money?

Thanks.

Try to hunt down an online manual for that router you ordered.... some of those models let you toggle the operating mode in the settings, like switching between Annex A and Annex B... if you get lucky with the one you picked.

So, is it really that straightforward?
If both options are available, there's an option to switch, but if that choice isn't there, does that mean moving to another annex is completely off the table?