RT-AX88U Pro Port Forwarding not working

[brentil]

Brand new RT-AX88U Pro
Firmware Version: 3.0.0.6.102_33320
UPnP Disabled

I've had a couple AC68U and AC86U devices so very familiar to the Asus world. I just replaced my AC86U with this RT-AX88U Pro and had them both online and used the one to be the basis to configure the other. Nothing else in the network has changed. I got everything else working except my one Port Forward for my Plex server. I went to the same place I did it in the old version, WAN -> Virtual Server / Port Forwarding and enabled it and added the port forward.

Plex - 44444 - 32400 - my static IP of the Plex server - TCP

And it doesn't work. I swap back to the old router and it does. I found an article on Reddit saying to instead go to Open NAT and do the rule there instead. That also didn't work either. Am I missing something "Pro" I'm supposed to be doing to make this feature function? lol

 

[brentil]

I figured out my issue, it had been so long since I swapped devices I forgot the AT&T Modem Passthrough requires you provide the MAC address of the router behind it. So, I was technically in a double NAT situation. Once I updated the MAC address to be that of the new AX88U Pro and rebooted, it got the public IP passed through and the Port Forward started working again.