Well,
the number of possible network setups and configurations _and_ how much a user knows about them makes this a bit too difficult to answer straight away

The keyword is "port forwarding", because at least somewhere in your setup is a firewall, blocking an incoming connection at that port.
From Windows own firewall, to 3rd party software aka "Internet Security" programs, to your routers firewall and beyond that even your internet providers modem might have a firewall running.
Which exact setup is used in your case and how to access and configure it would, in a best case scenario, involve someone knowledgable enough present at your computer, showing you how.
Or you can try your luck on the discord server, where a more direct communication is possible.
(Be aware though, the server runs highest authentication level, so you need to verify a cell phone number. That's discords way to keep it slightly more cosy

)
Also there's the networkplay channel to find players to connect with and/or test your setup.
See you there, maybe

K.
PS: The link is in the official Forge release post under "Feel free to jump on in and say hi!"
PPS: Check this out, maybe it works for you and your friend:
https://www.slightlymagic.net/forum/viewtopic.php?f=26&t=28909&p=238664#p238664