Problem solved, thanks everyone for their efforts !
The problem was that even if sitting on it it only produced 40Mb/s on client and on the MR33 itself.
Which was a lot different right after initializing the device for the first time.
Checked all the proposed settings, set Vlan from "0" to nothing but with no result. No traffic shaping or firewall rules established.
Ended up resettting to factory defaults, again Vlan to nothing and now its working just fine. Radio settings on max.
Strange enough it took about 15 minutes to reinitialize including temporarily vanishing of the SSID and beeing unable to reconnect by occasion. Seems like there was more to do than just restart?!
Troughput now 250/40 Mb/s on client and 210Mb/s on the MR while "sitting on it" with 1GB PoE injector and Netgaer 4port unmanaged up second floor. First floor still 140Mb/s. Basement 80Mb/s.
I am fine with that, now to see if there is a cure for that MX65W which is unwilling to do more than 250Mb/s.
Regards
Stef