The main problem: OpenWrt does not support UPnP out-of-the-box because it’s a big security flaw. This will lead to a “restricted NAT” on the PlayStation.
One method to get around this might be by Enable Full Cone NAT (See)
I already set full cone NAT at the ISP Router now. And I guess I need to make the same setting on the GLInet… Sadly no setting in the GLInet.
Yesterday I reset the firmware GLInet, and set the checkbox of Wireguard VPN back to be not accessible from the server. Additionally I took DMZ for GLInet in the ISP Router and DMZ for client (PS4) in the GLInet itself. (Noobs effort I dont know which will work hehe)
(I have set the MTU to match VPN too)
The connection seems back to business. Upload via VPN can reach 20 Mbps.
I think I want to cool down and hope the connection will be stable.