wifi with cellphone very slow

Check an see if your phone has any pending updates and attend to those. Run a speed test before and after changes to know if you are making any progress or not. 
The problem might be with the phone and not with the gateway. Run speed testing with another client on the wireless LAN to have a good comparison if you have not done so. 
If the gateway display reports at least 3-4 bars for the cellular signals that would be a good sign. Depending upon the phone age, capabilities, etc… which are not posted it is hard to really provide good direct suggestions. 

Welcome to Tmobile home Internet! You'll find out that it's probably not worth the money. From reading posts in the forum it's either really good or really bad.