RT-AX88U - WiFi 2.4 GHz client has degraded internet connection speed when the router has connected to the internet via 4G modem

[Dilbert2]

Hello,

RT-AX88U connected to the internet via 4G USB modem.

Strange behavior: if I connected to my router via WiFi 2.4GHz, speedtest from the client shows about zero. Speedtewst from the router iteslf shows the full speed.
If I establish a VPN connection between the router (as a client) and my remote VPN server, the same WiFi 2.4 GHz client has normal connection speed.
All other types of connections between clients (5GHz and wired) work well regardless to VPN connection.
QoS doesn't influence on the situation.

Does anybody have any suggestions?

 

[eibgrad]

About zero, or zero? There's a difference.

Almost sounds like that particular client is being directed through the OpenVPN client by the VPN Director, and the kill switch is active, hence no internet access is available to it unless the OpenVPN client is active. I know w/ the latest 388.8 release, the kill switch is persistent, even when the OpenVPN client is disabled/OFF.

 

[Dilbert2]

About zero. I mean that during some first seconds the client has some traffic, but in a few secs traffic is over. And the client in this state may be quite a long time (up to 30 mins). After that period some first requests will reach their target hosts, but after some seconds the picture will become the same and traffic will come to zero again.

The most strange thing is that the behavior above is for 2.4 GHz wifi clients only and only if the router is connected to the Internet via USB 4G dongle.

 

[Dilbert2]

And yes, FW is the latest one. Two previous versions have the same "feature".