It suddenly started to work. I'm not sure what was wrong, but it was ISP's router fault. It didn't properly take DMZ configuration, I tried setting it several times and restarting the router and finally it works now. It's "Huawei EG8145V5" and I find it very buggy :(
Anyway thanks for taking a...
System Log Port Forwarding logs:
Source Destination Proto Port range Open by Redirect to Client name Local port
ALL ALL UDP 51900 VSERVER 192.168.1.2 rpi4 51900...
I have Asus RT-AX53U and my setup is as follows:
Internet -> ISP fiber router with public IP and DMZ set to 192.168.18.2 -> Asus router with WAN IP 192.168.18.2 -> Port forwarding configured for VPN on 192.168.1.2:51900
This doesn't work, I cannot connect from the outside to VPN on server...