additional information, testing on my iPhone SE (all updated) and using both Wifi mobile net and LTE on the same t-mobile network. on wifi it works, on LTE it does not. again, it is on the same network by t-mobile, the difference being connected directly, or - using the same SIM - using the mobile net device. (I simply plugged the SIM into the pocket wifi).
therefore, it cannot be LTE VPN etc with the carrier. It has to do with the app thinking wifi vs. cellular.