Can't connect to a specific friend in any game

Aug 8, 2018
Can't connect to a specific friend. We both have the same ISP and live within 3 miles of each other. We can talk fine on discord or skype, but if we play a game we can't connect to each other. We've tried several PC games and a few PS4 games. We can connect to each other if there is a dedicated server. But we can't peer-to-peer. We can't connect to each other. This usually fluctuates. For a few months at a time we will be able to connect to each other with no problems then all of a sudden we can't for a few weeks, rinse and repeat. It's just annoying now and we can't figure it out.

We've tried port forwarding, UPnP triggering, DMZ on/off, disabling firewalls.

At this point any help would be greatly appreciated.
To make a connection to each other one of you needs to allow the inbound connection. the NAT is going to block this. the isp may be blocking it as well.
make sure you don't have two nats. a lot of modems have a router in them and then if you connect your own you have 2 nats.
port forwarding will pass the traffic to the client for the inbound connection.