In case this benefits anyone else, I've been using soracom.io with the nRF9160 in Canada (Telus) with no issues.
It didn't require any special configuration, just registered the card on console.soracom.io, plugged in the SIM to the Feather and fired it up. Within a few mins everything registered and I've been sending data to/from Amazon AWS Core IoT via MQTT Pub/Sub with OTA firmware upgrades. No issues and have been running 4 SIMS like this for a few weeks.
The Soracom console is fast and easy to use, and the price is right (7c per MB). Next step is running the nodes in the USA next week.