AT&T and TMobile use very similar technology (GSM). You would be better off dumping one of them (TMobile) and using Verizon which depends on CDMA. There are exceptions and overlap on their tech. Then get a cellphone booster. Having both technologies with a cell phone booster will minimize down time.
The following linked item will boost upto 4 devices at one time. They also sell cheaper single booster devices. The more expensive item does boost more signal strength. I recommend placing your mifi on or next to the internal antenna. I average 35db signal strength increase. The cheaper Sleek device I average 18db signal strength increase. With the booster I have cell and data service miles beyond other users.
I have Verizon. The CDMA tech works better in the mountains. The Verizon 4g is GSM. In July, I had Netflix where others had just enough signal to text.
Weboost (Wilson Electronics is the old name) does sell other antennas. However, every antenna has a downside. Different antennas can help 1x and 3g while hurting 4g. The included 4" rubber antenna is nearly indestructible and is mostly neutral.
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