Add more LAN Ports

Others here might be more insightful, but I believe what you need is a gigabit internet switch. Something like a TP-Link TL-SG108. Not recommending that one specifically, but it's an inexpensive option from a reliable brand. We have a couple of very old switches in our office that have worked w/o issue for many years. One is D-Link and the other is TP-Link. There are also 5-port switches if you only need to add a few extra ports.

Of note, I'm using the switches mentioned above on a separate network which is not connected to the T-Mobile gateway. So, I can't confirm that the switches work with the gateway, just assuming that they should.