Are NAT and UPnP green for both you and your friend?
- If NAT is yellow or red, you can lessen any restrictive NAT settings in your router;
- If UPnP is red, you should be able to turn it on in your router settings;
- If, for whatever reason, both you and your friend have all of those green, look up online if your internet provider is using Carrier-Grade NAT (CG NAT). If this is the case, you can usually call them and ask to be removed from behind that. It hides you by sharing an IP with other people.