devices on Wifi

I do not even have any statistics for 5g and I  about a mile from a tower. My house has old aluminum siding and I am wondering if that is hindering somehow. I am thinking a mimo external antenna will help. 

Take the gateway outside and then check the metrics. T-Mobile support can tell you what cellular signaling you should expect. Confirm that the tower you believe to be the source for the network is correct. Yes aluminum siding could be part of the problem. If you are not seeing 5G then you should contact support and get some verification on the cellular signal and location. 

Thanks for the advice. It is raining outside so I will have to possibly wait until the weekend to try this. I get 5G as soon as I get to the top of the hill on either side of my house. 

A hill you say? Humm that doesn't bode well. If you have to go to the top of the hill to get 5G service that might just be a rather big problem. I think the house is in the wrong place. 
I live in the western ridge and valley part of the Appalachians and I can see the entire tower 5.3 miles away so it works but hills present huge problems for valley dwellers. An external 4x4 MIMO panel antenna might be your only alternative. Check out Waveform.com for their prices. It is not an inexpensive endeavor but it does help some people.