Ports are opened so that shouldn't be the problem.
I have the Router from my ISP set to forward the Port to my Main Router (1st Screenshot) and then they're forwarded to my PC (2nd Screenshot)
253.903 Hosting game at IP ADDR:({0.0.0.0:34197})
253.903 Info HttpSharedState.cpp:54: Downloading https://auth.factorio.com/generate-serv ... _version=4
254.502 Info AuthServerConnector.cpp:69: Obtained serverPadlock for serverHash (7GRWXCJ7PFSBI7MXnXtVyAV6lhD1pO7g) from the auth server.
254.502 Info ServerMultiplayerManager.cpp:783: updateTick(2643307) changing state from(CreatingGame) to(InGame)
254.503 Info ServerRouter.cpp:646: Asking pingpong servers (pingpong1.factorio.com:34197, pingpong2.factorio.com:34197, pingpong3.factorio.com:34197, pingpong4.factorio.com:34197) for own address
254.515 Info GameActionHandler.cpp:5007: UpdateTick (2643307) processed PlayerJoinGame peerID(0) playerIndex(0) mode(connect)
254.630 Warning ServerRouter.cpp:521: Received own address message reply with conflicting address (got IP ADDR:({212.95.5.173:56518}), expected IP ADDR:({212.95.5.173:54312}))
254.630 Warning ServerMultiplayerManager.cpp:628: Determining own address has failed. Best guess: IP ADDR:({212.95.5.173:54312})
254.663 Warning ServerRouter.cpp:521: Received own address message reply with conflicting address (got IP ADDR:({212.95.5.173:30482}), expected IP ADDR:({212.95.5.173:54312}))
254.797 Warning ServerRouter.cpp:521: Received own address message reply with conflicting address (got IP ADDR:({212.95.5.173:49060}), expected IP ADDR:({212.95.5.173:54312}))
254.801 Info SteamContext.cpp:486: Lobby created callback started.
254.801 Info SteamContext.cpp:493: Lobby created succeeded.
254.801 Info SteamContext.cpp:515: Lobby creation finished.
254.801 Info SteamContext.cpp:523: Entered own lobby.
255.227 Info MatchingServer.cpp:114: Matching server game `9936277` has been created.
255.230 Info SteamContext.cpp:475: Leaving lobby...
255.231 Info ServerMultiplayerManager.cpp:712: Matching server connection resumed
255.496 Info SteamContext.cpp:486: Lobby created callback started.
255.496 Info SteamContext.cpp:493: Lobby created succeeded.
255.496 Info SteamContext.cpp:515: Lobby creation finished.
255.496 Info SteamContext.cpp:523: Entered own lobby.