The ISP did the port forwarding in their router (since they have remote access to it). Because of CGNAT they don't allow their users to mess around in their router settings.
The server does work on my Asus router built in OpenVPN service, it's just that any device behind my Asus router that...
My Asus router (flashed with Merlin firmware) connects to my ISP router via static IP because my ISP blocks their router config, so I cannot put it on bridge mode.
I also have NAT on because I do not trust my ISP’s router security.
Since I needed to create a VPN server, I had to call my ISP to...