You must forward port 7777 (or port you entered in DefaultEngine.ini) on you router. Not on you PC. It’s named “Port Forwarding”. You must open input port on router, then client from the Internet can connect to you. There are two technics to do that programmaticaly from you game on your PC in local network: UPnP and NAT-PMP. Better practice - use combination of technics, e.g. when you router denied access to UPnP or doen’t have this, you can try use NAT-PMP. And opposite situation. This technics in combination cover about 96% routers. Other 4%… Well… You can display instruction screen (for manual port forwarding) in your game. 
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