And a follow up. I tired setting it back to 300Mbit/40 MHz and started a file transfer, after around 600-700Mb the link between the Access Point (TL-WA901N) and the repeater (WA850RE) stopped...

I can confirmed what you wrote. Changing channel and setting the speed down to 130Mbit (20 MHz) the wireless is stable. But that is not what I expected of this product. My primary access point is a...