openWrt wifi slow when Raspberry PI 4 with OMV connected


I've the latest release of openWRT on a Netgear router (WNR 2200) and it worked fine.

Just recently I bougth a raspberry pi 4 device and installed openMediaVault (NAS software) on it.
Then I connected the raspberry to the openWRT router using an ethernet cable (no wifi connection, just the cable).

After a while of just files browsing and two files successfully copied, the wifi (managed from the openWRT router) becomes very slow (less than 500 Kbit), almost useless, even if no file transfer is going on.

I connect via SSH to the raspberry and send a "shutdown" and suddenly the openWRT wifi goes back to 50 Mbit!

It seems that openWRT is influenced by the raspberry (with OMV) in some ways, but I wonder what the problem is and how to investigate this issue between the router (openWRT) and the Raspberry..

Sorry for the few infos, I need some hints on how to investigate the issue

Thanks in advance.