I can confirm that I experienced a similar issue on my 55UJ6300, and apparently changing the WiFi RF module worked. I was not using WiFi at all, since I had my TV connected by Ethernet cable to the router, but around November 2018 I started seeing the "Unknown device disconnected" pop up messages. Some days it would happen more often than others. Sometimes a "Wired cable connected" or similar message would also show up, and sometimes a bigger black notice would show up saying "Network connection lost" or something similar. Sometimes the Ethernet connection would actually stop working after this. I also started seeing the 2-3 second pause followed by fast motion to catch up every once in a while. This would happen for example when watching YouTube or Netflix on the TV apps. Netflix would sometimes require me to get out of a movie and back to it for it to resume playing. Turning on Store Mode would hide the pop up messages, but I am sure they were still there only hidden, since the othe symptoms would persist.
As someone in this thread suggessted, I opened the back of the TV, looked at the RF module, and found a similar one on eBay for less than 10 dollars (here in the US). The module was the same though the revision was a bit higher (1.1 in mine and 1.4 in the one I found). I chose it since I figured maybe a newer RF module would work for a longer time, or had the issue fixed so that it will not happen again in the future after a while. I was able to successfully switch my module with this one, and even though I have not tried WiFi directly, everything I needed seems to be working, and all the issues have disappeared for now. It has only been around 3 days, but I am hopeful that the issue will not come back.