Wireless Joint

I'm not sure I follow exactly what you are trying to accomplish, but ideally what ever device and modem you are using for service should have both antennas outside. There is LTE antennas you can get that have two antennas in one enclosure. This is typically called a MIMO antenna in advertising.

In my many experiments i typically find that only having one antenna attached to the primary antenna port on the modem cuts the download speed in half.

This is assuming a lot of variables in your location, distance from the tower, etc, but if you are running in the highest transmission mode in LTE (this is something you can't control the modem and the cell figure this out automatically) then you'll more than likely get double the speed.

It's also not just about spatial multiplexing, if you are in a place where signal is really poor then having the second antenna helps with error correction as the modem will run in spatial diversity mode and you'll likely see less packet loss and a more stable connection (but maybe not more speed.)

I know wilsonamplifiers. They have competitors. I bought - https://www.myamplifiers.com/repeater-20/
It supports all mobile operators in the UK.
Easy to install. Works good
Two antennas, convenient mounts and a long cable, 12V power adaptor.

I have an Ubifi setup at my cabin and went through similar issues as you minus the connection instability but it sounds like you got that one sorted. I think it's very strange that Ubifi doesn't clue you in that you need 2 of their overpriced wilson antennas to make an effective setup. In my case I had also ordered one antenna with 75 feet of cable and I had 20-30mbps but my signal didn't improve much at all over the antennas that come with the device. I ended up finding this forum and ordered 2 of the https://thewirelesshaven.com/shop/antennas/4g-lte ... l-antenna/ and 6 ft of cable and have a slightly improved signal and average probably 50mbps now. I am very much crossing my fingers that the https://thewirelesshaven.com/shop/antennas/700-27 ... l-antenna/ antennas come back in stock soon as I have heavy leaf cover in my area and hoping they make a difference. Also I can add that Ubifi doesn't want you to use their sim cards in another device other than a Mofi 4500. I have a feeling that At&t would disable your card if you tried but it sure would be nice to know for sure.