Arcadyan KVD21 5G which connectors for 5G external antenna

If you can get the cellular metrics in more detail understanding just what is going on might help a bit more. The speeds in the history are pretty inconsistent. Do you have the metrics for both the 4G & 5G signals such as the band and the bandwidth in MHz for them? I suspect in some of the cells if the bandwidth is less than 20 MHz and user load/demand is higher that contributes to less bandwidth per user session. Then it is more critical to have a clean signal to prevent packet loss that will result in retransmissions. 
Use cellmapper.net to confirm the locations of other towers/cells in the area as you consider antenna placement in relation to the tower you are using. Here I know there is noise from other RF sources towers the west/northwest. I have my gateway in a window with northeast exposure to get the signal from the tower that is due north just because the signal is cleaner than using the north facing window. Results are better so I know using the house as a shield can help. The other obscure issue can also be with reflected signals. Cellular signals seem to be subject to multiple external factors so it is tricky at times to avoid issues.