advice: returning to stock etc topic

I gave my mom my old HTC ONE M7, which she loves. It started having the audio problem recently in calls, and I couldn't fix it over the phone troubleshooting, so I had her send it to me. To make things easier in the future, I decided to return to stock so it could get OTA updates in the future. It is unlocked with s-off and I had a custom ROM on it for her.

It had a 1.55 HBOOT version and I read that I should flash down to 1.44 HBOOT and then RUU (RUU_M7_UL_JB_50_Cingular_US_1.26.502.15...etc) to stock image which I have done.
The phone is now running AT&T stock:
hboot: 1.44
firmware: 1.26.502.15
model: pn0712000
cid: 11111111

So now here's my particular issue. I don't have a sim card that will fit into the phone. My mom lives far away, she kept the card to put in her backup phone while I fixed this one. I can't update the phone on AT&T's network without a sim card, so I can't do OTA updates to get it up to date.

I have done some searching but found nothing definite. Are there no downloadable OTA files from AT&T? If so, i'm assuming every update must be done in order (in other words, i can't just find the latest OTA and apply it, they would need to be applied incrementally)?

Anyone have any suggestions?

If I was to forget about AT&T and go to straight HTC image or Google Edition, (where it could still get OTA) what should I do from here?

I suppose if I can't find anything at all, I can go to an AT&T store and see if they can let me use a sim card or something (I have AT&T as well, the same account as hers, but my M8 uses a smaller sim card).