66 Is the primary prioritization band for T-Mobile, yikes.
I saw the 8800L, but those ports are low-pass test ports. Right now, the most recent Jetpack series that I can find that has an antenna port is the 4620L.
That would just be added to the T-Mobile hotspot available in the car from the Anysecu 4G-WS mobile network radio, and a Sprint Netgear Zing hotspot. Both of those access the same network, just through different bands, with the Zing relying on the old Sprint 25/26/41 slate. I would truly be up in the air on a backup hotspot, whether to go with Verizon or AT&T, but I only plan on making that decision if I do any work in the Plains. Since the merger, I haven't had any problems locally with the Sprint/T-Mobile combined network.