I have one mavic on the 200 firmware and the other on 800 firmware for the goggles. I rarely have issues unless I’m in a high WiFi area. Or flying near the power plant near my house. Try the 200 and the 800 firmware and see if that helps. Heavy wooded areas could also be a problem if not high...
I did this back when they first came out. There are many threads about this and I also posted about it back then. Just saying and they did fix this problem. Maybe he got some old stock from some place..