Check out Coverage Map app by root metrics. If T-mobile and ATT are close in coverage, go with T-mobile. If not, consider picking up a real cheap Gophone, even a used ATT smartphone, activate it, then use the IMEI. You will have to call ATT and ask advanced tech support for help with this.
Its a pain in the neck, but if you need coverage, you gotta do.
(FYI oneplus phones have no CDMA bands which are required to work on Verizon and Sprint. ATT, T-mo and the rest of the world are using GSM technology. )