DIAGNOSIS The Linksys app simplifies a lot of advanced settings. For port forwarding, you need to use the traditional router web interface, which the app doesn't fully expose.
FIX You have to access the router's dashboard from a web browser. Here's how: 1) Make sure your computer is connected to your Atlas Wi-Fi. 2) Open a browser (Chrome, Safari, etc.) and type 192.168.1.1 into the address bar. 3) Log in with your router's admin password (this is the one you set during initial setup, not your Wi-Fi password). 4) Once in, look for "Connectivity" or "Router Settings" in the menu, then find "Single Port Forwarding" or "Gaming" (which often has port forwarding rules). From there, you can add your rule: give it a name, enter the specific port or range, and the local IP address of the device (like your gaming PC or server) that needs the ports open.
IF THAT DIDN'T WORK If 192.168.1.1 doesn't connect, your router might be using a different subnet. On your connected computer, open a command prompt (Terminal on Mac) and type ipconfig (or ifconfig on Mac). Look for "Default Gateway." That IP address is what you type into your browser.
HEADS UP Port forwarding is a security risk if you don't know what you're doing—you're opening a door into your network. Only forward ports you absolutely need, and only to devices you trust. Also, for the rule to work, the device you're forwarding to needs to have a static/reserved IP address on your local network, otherwise the rule will break when its IP changes.