Too Slow

It seems that some TVs don't like the default network on the T-Mobile gateway.  The default network has both 2.4 GHz and 5 GHz bands. It also has the highest security.   My suggestion is to add a second network using the + at the bottom of the Network tab in the app.  Set the second network to 2.4 GHz and WPA/WPA2 security.  Set your TV to connect to the second network and tell it to forget the primary network.  Only use this second lower security network for devices that aren't working well with the primary network.

If you really can't get acceptable performance with T-Mobile internet you should keep Spectrum.  When I called Spectrum to cancel, after setting up T-Mobile, they offered me a significant discount.  I didn't take it because T-Mobile is working better for me.  You could tell Spectrum that you are dropping them and see if they offer you a discount.