Talk to your T-Mobile support folks again and tell them you cannot use wifi DFS channels.
The nine 5 GHz channels supported by Roku devices are 36, 40, 44, 48, 149, 153, 157, 161, and 165.
The channels assigned to the 5 GHz band in ranges 50-64 and 100-144 are shared with other uses such as military, radar, and weather. These channels are termed DFS (dynamic frequency selection) channels and routers are supposed to automatically switch to a different channel if they detect one of these channels is in use in your area. Roku and many other home use devices don't support these DFS channels.
Many home routers don't include DFS channels, but some do. Make sure your router is not using these DFS channels, and that it is not using an auto select option that could choose a DFS channel.