My buddy and I just did this yesterday: we flew together via multiplayer just using only native XP11 over the internet. It works beautifully and once established it is always there, provided of course that the internet is up and you don't disable it.
Based on my memory, here is what we did:
(1.) Determine each players public (Internet) IP address
(2.) Configure your routers to "PORT FORWARD" ports 49000 and 49001 from your internal ip address (192.168.0.xxx) or whatever....
(3.) Configure the Windows Firewall on your computers to allow all traffic through the firewall from X-plane
(4.) On the NETWORK setting tab in X-Plane11 do the following
(a.) Check that the UDP ports match what you set in your routers' port forwarding (i.e. 49000-49001) on the UDP Ports menu
(b.) Enter the others' IP address on the MULTIPLAYER menu and then click "Add Multiplayer Machine"
(4.) Both of you setup the same flight in X-Plane 11
(5.) Check in SETTINGS on the NETWORK tab: If you did everything above correctly, you should see indications under "SENDING DATA TO" and "RECEIVING DATA FROM" sections (for example: "Multiplayer machine 1 Cessna Skyhawk (xxx.xxx.xxx.xxx) {Your buddy's IP address } ...... Just now" that you are sending and receiving data with your buddy.
(6.) A quick test is to send each other a text message from within the cockpit by hitting the ENTER key, typing a short message and hitting ENTER to send it. Your buddy should see it pop up on a single line in the middle of his screen.
(7.) One note, if you both start at the same default position (i.e. rwy start position) you might not see each other, because X-plane has a tendency to put one plane on top of another!
Like you, I am brand new to Xplane11 and coming from FSX and DCS. This was a pretty easy setup, but didn't find very many tutorial's on it.
Hope this works for you.
Alan