I tried this software in 3 computers with Ubuntu 18.04. All of them are within the same wireless LAN and I noticed that there is a pretty noticeable lag while moving the mouse on the 2 computers that have Barrier working as client.
Try it with Ethernet. Especially, if some devices are too far away.
The degradation of the connection’s quality can be very extreme with WiFi.
P.S.: WLAN signal repeaters are pretty much a marketing gag, because they simply amplify the signal at the point they are installed at. So if you repeater is installed far away from the router, then the signal coming from the repeater won’t make it much better, because it will only amplify the already very low quality of the original signal.
That sounds pretty good. So perhaps the WLAN may not be an issue.
The problem with your situation is, that there could be tons of reasons responsible for the issues you are experiencing. So the debugging process for your situation could perhaps take a very long time and a lot of effort.