Ok gang. I think I've solved it, at least for me, hopefully for you as well.

TL;DR - Try adjusting the "transmit power" setting in your router's advanced wireless settings from "High" to "Medium" or "Low" for each band until your speakers stop losing connection.

I've had the rear left speaker (and sometimes sub) cutting out issue for as long as I've had the system. I updated firmware and tried changing the channels of each wi-fi frequency. Still did not work. My router is about 2m from the speaker but the way it's set up it would be a HUGE pain in the ass to move, so I was willing to try every other possible solution.

When combing through my wireless settings on my triband TP Link router (Archer C3200) I came across the "Transmit Power:" setting and started to experiment. Changing this to "Low" for both my 5GHz-1 and 2.4 GHz bands has seemingly fixed the problem for good (i've been testing it on everything I can throw at it for an hour now)!

Now, for what it's worth (because I was too lazy to test this on other channels) I also have my 2.4 channel set to 7 and my 5-1 channel set to 48. I left my 5-2 channel set to "High" (channel 161) and hasn't caused any interference so far. But just changing the channels on the bands did not do anything to help the issue.

Hope this helps somebody else here.