I’m quite handy with linux and networks, and it was only by chance that i found out what the problem is.
For me it was the router using a tunnel to bundle wired internet and mobile internet.
That reorderd the packet i believe creating the problem.
Anyway if MLAT is still working and maybe even if not, this kind of thing is really hard to debug.
So it’s probably not worth the effort.
The lowest effort would likely be, trying an ethernet cable to exclude the WiFi connection as a cause.
Or just turn the RPi on it’s side or the other side.
See if that helps with WiFi reception.
The RPi3 has a very bad antenna.