Asus RT-AX53U - double NAT and port forwarding

[Acid5700]

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 192.168.1.2 which is connected to Asus router. I tested other routers (e.g. Xiaomi and TP-Link) with identical setup and it works there as expected, so it's not a matter of incorrectly configured firewall on 192.168.1.2 server.
What could be wrong? What else I can check to make it work?

 

[ColinTaylor]

Verify your port forwarding rule is enabled under System Log - Port Forwarding.

Turn off DoS protection in the router's firewall.

Are you using TCP or UDP for your VPN?

 

[Acid5700]

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

DoS protection is already disabled in firewall settings.
I am using UDP for VPN

 

[ColinTaylor]

Thanks for the info. That all looks as it should so it ought to be working.

Do you have a device on your network with a TCP interface, like a network printer or SSH server? For testing purposes you could setup a TCP forwarding rule for that and see if it is accessible from the internet. I'd suggest using a non-obvious external port mapping for this test. At the very least you would be able you use the online tools like yougetsignal or canyouseeme to test the connection which is not possible with UDP. Alternatively, temporarily change your VPN server to TCP with the associated change to the port forwarding rule.

 

[Acid5700]

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 look at this thread!