Diagnosis
ASUS moved the port forwarding settings in their newer firmware. It's now under 'WAN' > 'Virtual Server / Port Forwarding', but it's easy to miss.
How to Fix It
Log into your router. On the left-hand menu, go to Advanced Settings > WAN. You'll see tabs at the top like 'Internet Connection', 'Port Forwarding', etc. Click the 'Port Forwarding' tab. Here, click 'Yes' to enable it, then add your rule: Service Name (anything), Port Range (e.g., 25565), Local IP (your device's IP), Local Port, and Protocol (TCP/UDP/Both).
If That Didn't Work
Make sure the device you're forwarding to has a static IP address assigned, either on the device itself or via the router's DHCP reservation (LAN > DHCP Server). If its IP changes, the port forward will break.
Heads Up
If you're trying to forward ports for gaming or a server, also check the 'Firewall' section (under Advanced Settings > Security). Ensure the general firewall is enabled but that no specific rules are blocking your traffic. Some security features like AiProtection might have their own game-specific passthroughs worth looking at.