Arcadyan KVD21 5G which connectors for 5G external antenna

You might check out Nater Tater's YouTube videos on the antenna addition. I am more familiar with the Nokia gateway and the waveform external MIMO antennas. I believe his videos focus on those both 2x2 and 4x4 MIMO. To take full advantage of the n41 mid band frequency a 4x4 MIMO is really needed. I do know Waveform.com has some excellent documentation for the gateway break down and the antenna additions. You might have to experiment with the connections a bit if it is not clearly documented in a way that makes sense for that antenna. One thing to be extra, extra careful about is making the connections to those u.fl connectors. They are tiny, delicate and easy to break. If you break one they are not too difficult to replace but just need the tools and the connectors which also can be bought online. 

Without any documentation I can't say which connections to make. I believe Nate has also covered the Sagemcon as well as the Arcadyan gateways. The Nokia he went over before those. The information from Waveform is excellent.