Carriers for Boron LTE?

Hello everyone!
In the USA, is there really only one carrier (AT&T) that provides a network that is compatible with the Boron LTE? The Boron’s info page says that it supports CAT-M1/NB1, and I see that there are other networks such as T-Mobile which support other NB1 devices, (haven’t looked for CAT-M1 as much since it’s hard to search specifically for that when there is also CAT-1) however it seems that people on particle community have had issues getting other carriers to work, and the above-linked particle page lists AT&T as the only carrier in the US. Does anyone have any experience with this? If so I would greatly appreciate some input/clarification.

AT&T is currently the only carrier that works with the embedded SIM. You can use a third party SIM on the Boron LTE (NOT the B Series) and use the carrier of your choice. I use Hologram, which in my area alternates the Cat-M1 connection between Verizon and AT&T. Search here for third party sim and you’ll find lots of info.

Thank you for the input! I have had borons running well with AT&T using external sims and their Cat-M1 network, but from what I can tell, it seems T-Mobile does NOT have a Cat-M1 network (they do have Cat-1) but they DO have an NB1 network which people have been using the SARA-R410 modems with successfully in other applications. I guess I am wondering if the boron is unable to connect to NB1 networks even though it's modem supports it.
From the boron's page here:

The Boron LTE is a powerful LTE CAT-M1/NB1 enabled development board...

I can confirm that T-Mobile does not have a Cat-M1 network after speaking with them. I haven’t tried the Boron with Cat-NB1 so I can’t say how well it might work with that. T-Mobile is banking solely on Cat-NB1 for IoT.

While reading some of the therms of use here in Germany, I don’t thing NB-IoT is a good thing for the particle devices. E.g. t-mobile ‘can restrict’ the service if 500KB of traffic per month are exceeded (also you need your device being certified?). By using a third party SIM you almost always need a ~40 second keepalive to maintain the cloud connection what results in about 200KB of traffic PER DAY just to keep connected to the cloud.
As far as I understood Telefónica (O2) has a LTE-M network here but all services (doesn’t matter if NB-IoT or LTE-M) are only available for large scale business-customers. Maybe particle has some chance to get access to these networks (my 3G-Boron uses Telefónica all the time so maybe there’s some kind of relationship).