T Mobile 5G Gateway very slow

If you only receive signals that provide 2 bars  on the LED screen the location is not covered well for cellular signals. The number of bars does reflect signal strength and 2 bars is pretty weak. If you want to improve the cellular connection you could possibly do so with an external antenna. It takes some technical expertise so is not for everyone. You also MUST keep in mind if you go the external antenna route and damage the gateway you are on the hook for the cost of the gateway or paying to have it repaired. Adding external antennas is not supported by T-Mobile. IF you cannot get more than two bars that is one way to have a solid chance of improving the cellular signal. Waveform.com has solutions and provide information about their external antenna solutions. You can call them or chat with their engineers and they are very helpful. The solutions are not cheap and run from around $250 to $450 depending upon the solution that is required. They can explain the 2X2 MIMO and 4X4 MIMO antenna solutions and why you would want or need one over the other. 

There are cellular "boosters" but they are more expensive for good units and not designed for use with the home gateways. The cellular boosters will amplify a weak signal and broadcast it local but the range and capability is limited and not going to do the MIMO without spending much more. Not a solution with the home internet gateway. The 5G cellular boosters are much more expensive than a 4G cellular booster so it is an expensive solution to ponder.

The wireless extenders will do squat for you without a cellular signal to connect to. The extender can extend the local wireless to areas farther away from the gateway that are hard to cover but will do nothing for the cellular signal. My guess, based upon the statement, "if we have multiple devices signed on, the WiFi fails" is more about the number of devices trying to get internet connection leading to even less uplink bandwidth for say just one device that is hampered by a poor uplink. If one device is fighting to get a session on the cellular uplink then more clients trying to get bandwidth for sessions will just be a battle and everything comes to a crawl or falls flat.

Locating the tower and getting a stronger signal for cellular is the first task. If you can't get more than 2 bars without an external antenna then it is probably not going to be a satisfying solution.