Hi,
Then if I'm understanding things right ( I've not done multiplayer online since mid 90's ) you will need to know your ip address which is visible to the outside world, you may be able to find this by logging into your modem hub.
Last time I was online it was via the Microsoft gaming zone.
This is the website suggested by my isp to find out the ip address visible to the public:
https://www.whatismyip.com/
But if you reboot your modem this will most likely change unless your paying for a static IP address.