Arcadyan KVD21 5G which connectors for 5G external antenna

I have no experience with the smaller antennas like that so I have no idea how much that will improve the reception. In the office we used an indoor 4G LTE booster. It helped but still was quite limited in capability as the only real way for the signals to get into the location was via a hallway to a door. The upper floor and all around our desk space was pretty much lab equipment or other electronic gear and upstairs more of the same. In a home without the heavy interference it would be a different equation. 

If you receive the midband then go for the 4x4 MIMO if it still fits the budget. If you only get the n71 then the 2x2 MIMO should be ok. I would probably shoot for 4x4 MIMO as you never know when T-Mobile might just upgrade the cell from n71 to n41. I did not expect it here as we are in a rural area with lots of pastures and fields but they did. That was just a little over a year from the initial n71 stand up. If they see growth and are planning for more users they will bump the cells up to n41 as the frequency can carry more data being a shorter wavelength.