The OP has a 4 year $450 prepaid subscription with T-Mobile so even if he was in an area that qualified for updating the modem, AT&T wouldn't be a good financial move since there are no refunds from T-Mobile.

I was responding to Cadx6. He didn't say if he prepaid for his T-Mobile subscription. Anyway, I prepaid for my 30 months T-Mo subscription as well. It's still active actually, but that didn't stop me from switching. It's just money. Why suffer through a bad experience? If I had known that at some point I would be able to switch to AT&T, I wouldn't have prepaid for a whole 30 months, but hindsight is always 20/20. On the other hand $450 is the equivalent of about 2 months of fuel for me, so it doesn't put much of a dent in the TCO.

I'm now using US Mobile on the T-Mobile network and in my area it works fine. At $4.70 per month I'm satisfied and not wasting money which, for me, is a valuable, non-renewable resource.

Update: I called the Audi Connect 800 number and they said the dealer only set up a 6 month trial which expired. I looked at my contract and sure enough it was for just 6 months. My bad. So, long story short, the A6 I traded in for this beauty had a couple of months left on it (I forgot to transfer it) so I had them move the remaining time left to my SQ5. All good until the end of Jan at which point I have to decide about re-upping with T-Mobile or giving the AT&T version a try.

If by hardware exchange you mean the data modem replacement to accommodate T-Mobile's "upgrade" of their systems in certain areas, I have done that on my 2013. When you started using the AT&T SIM from a tablet, did you have to give AT&T a new IMEI number, or did you just "stick it in" and it worked with AT&T thinking it was a tablet?

If you want to use AT&T and already have AT&T as your phone provider then you need to get a new sim card and add it as "iPad" so you can use the mobile share plan. They will need the IMEI, which their system won't recognize it, but just make sure them add it as "iPad" and yes you're getting 3G service and better coverage than T-mobile for $10/month.

No, I didn't give AT&T a new IMEI number. As far as they are concerned, my MMI is a tablet. However, since you have done the hardware exchange, you can call 877-505-AUDI Option 1 and officially sign up for AT&T service. They will send you a new SIM matched to the IMEI of your MMI.

Thanks thanking & superswiss very much for the info.
I currently have an iPad on my AT&T Family plan. As a test, I think I'll try putting the SIM from that in the Audi (with an adapter, naturally) & if the performance is measurably better in this area I'll call the number you provided & have them send me a SIM & activate it on my account.
I still have 6 months left on my prepaid T-Mobile plan but if AT&T is faster with better coverage, it will be worth the sacrifice.