Who is on my network

magenta8135852 wrote:

How can I find out who is connected to my T-mobile gateway> 

Use the T-Mobile Home Internet app, Devices.

I use TrendMicro HouseCall for home networks. Gives lots more information than just in the router table. And it's free. 

https://www.trendmicro.com/en_us/forHome/products/housecall/home-networks.html

If, as many have found, the T-Mobile app or gateway web interface don't give you the info you want, a  Google search will find you a number of free and paid apps for doing what you need. Just be careful to read through all the licensing and privacy boilerplate. Most of the free apps collect data about your networks and devices for their own use, and that is an issue for many people.