OK I tried this now: Only one wireless connection is active, which is the 5GHz connection to the FritzBox. I set the RUTX10 to "Force 40MHz mode" on the 5G. I think this is what you mean? - Situation is the same, downstream full speed, upstream very below 0.1mbps, while the internet connection can do above 10mbps upstream. Signal strength in the RUTX10 is shown above 80%.
The FritzBox doesn't allow channel bandwidth to be set. I tried a couple of channels instead of the auto setting (which had picked channel 40), no change of situation. Also, I live in a house that with not too much around it, there are now other 5GHz networks around.
I tried lowering the MTU on my Laptop, made things not better, rather worse. Tried lowering the MTU on the RUTX10's WWAN device… upstream speed test drops to 0.00mbps.
Also I logged into the CLI of the RUTX10 and ran "speedtest" – which wouldn't yield a better performance. The difference to running speed test on my Laptop is supposedly, that no routing in the RUTX10 happens, since it would just connect via its WWAN interface IP.
So there really is an issue with the WLAN in client mode in my RUTX10 or with the device in general, maybe a bug/misconfiguration that cannot be fixed using the GUI.
First job with supposedly that configuration comes closer… getting nervous.
EDIT/UPDATE: It works without any issues with other FritzBoxes, I visited relatives and friends today and tried. I'm busy the next days, but probably a firmware update of our FritzBox at home could fix this issue. So, the RUTX10 is alright, great little rugged box that works as designed.