We all want configurability, passthrough/ bridge mode to use our own routers avoiding the dreaded double NAT that can effect network performance. The problem is not with T-mobiles gateway’s. It has to do with the network. All of the home and business gateways don't get their own public IP address. Users in an area are sharing one public IP address. Unless T-Mobile will allow for static IP addresses is why these gateways have limited configuration options. Im not sure if its an option. I haven't called tech support to ask if a static IP is available. This is why you will see slow downs at peak usage times.
In layman's terms, picture 10 devices on your home network all streaming 4K video with a 150 MBPS connection, it can't handle that much bandwidth. Same goes for the T-mobile home internet modem/router but the problem is your sharing the connection with multiple Home internet customers.
If you have a cable, fiber or a DSL connection you do in fact get your own public IP address. Cable connection is the is lets say a 2” pipe and everyone is feeding their 1/2” pipe. Slow downs happen at peak usage. DSL is limited to the copper infrastructure as far as the amount of bandwidth it can provide along with the distance from the central office (CO). Fiber is different That is why you can get full duplex speeds (same up/down) because a light signal doesn't degrade as much. Very rare that you get slow downs with fiber.
There is optional devices from other companies that will get rid of the DHCP which causes network and double NAT issues but the fact still remains you're still sharing one public IP address with multiple clients.
I hope this makes sense to some people.