A backer asked on Kickstarter hence posting here.Ting (GSM) uses T-Mobile and is fully compatible with Teracube. However - their compatibility checker is conservative and will not list any new phones like ours. Here is some more info on their network:
https://help.ting.com/hc/en-us/articles/205428788-Buying-or-bringing-a-phone-to-Ting
(Expand the GSM phones and tablets section ; the network info is fully compatible)
I used Ting mobile for a few months near Boise ID and I used Sprint towers.
Are you sure you were using Ting (GSM). My understanding is that they offer 2 different SIMs - GSM and CDMA?
I originally went with ting because it offered sprint towers. I wasn’t asked what network I wanted to use when I signed up.They might give you a TMobile SIM card if you request it.
I have been using my Teracube on Ting for several days now. I was able to pop the GSM SIM from my old phone in and was immediately connected, no problem.