Diagnosis
The port forward rule is not applied, or the server PC's local IP address has changed, breaking the rule. The rule maps an external port to a specific internal IP and port.
How to Fix It
First, assign a static IP to your server PC. In the router interface, go to Advanced > Network > DHCP Server. Find the 'Address Reservation' section. Add an entry for your server PC's MAC address, reserving a specific IP (e.g., 192.168.0.150). Save. Second, verify the port forward rule. Go to Advanced > NAT Forwarding > Virtual Servers. Add a new entry. Service Type: select your game or use 'Custom'. External Port: the port the game uses (e.g., 25565). Internal IP: the static IP you reserved (192.168.0.150). Internal Port: same as external port. Protocol: ALL. Status: Enabled. Save. Reboot the router.
If That Didn't Work
Your ISP may be using Carrier-Grade NAT (CGNAT), which prevents inbound connections. Check your router's WAN IP in Advanced > Status. If the WAN IP is in the range 100.64.x.x to 100.127.x.x or is a private IP like 10.x.x.x, you have CGNAT. Contact your ISP to request a public IP.
Heads Up
The Windows Firewall or other security software on the server PC must also allow the connection on the specified port. Test the server locally using 127.0.0.1:PORT first. Port forwarding will fail if the server PC is in sleep mode or powered off. The router's 'NAT Boost' feature must be disabled for some port forwarding scenarios; if problems persist, try disabling it under Advanced > System Tools > System Parameters.