OK, finally got all the parts and cables to do this and again, thanks for the great write up how to on this. The process was flawless.

I am in a condo so I am unable to cut holes to the outside or put anything on the roof so I went with a smaller Netgear antenna and thinner SMA extension cable to cut a smaller hole into the window frame to sneak the antenna outside and stick to my clapboard. Don't think the condo police will notice.

Antenna I got for $19 on Amazon: https://smile.amazon.com/gp/product/B07K143JS5/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

And the 2 SMA extension cables for $6 each: https://smile.amazon.com/gp/product/B07Q7174DR/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

At the moment, the flakey T-Mobile Gateway is only giving me the primary LTE channel. The secondary 5G channel cuts out randomly whenever I start thinking it will continue to work right. With just the primary LTE channel I was getting single digits using just the internet Gateway antennas. With the new antenna outside, I am getting 50-80mbps down and still single digits up but that is a very good improvement and makes the service usable on just the primary connection.

When the secondary 5G channel works, I was getting a good 100+mbps down and average 40mbps up. I am anxious to see if this new antenna helps the 5G as much as the LTE connection. I'm giving T-Mobile until July to get that secondary connection on the 5G more solid. If they can't or won't, I may scrap the entire setup and go back to the evil Comcast. Hopefully I won't have to.