Sending data from one PC to another on different networks without the need for a server / 3rd party app

I wanted to know what this subreddit thought about this problem. I spend all day scratching my head at it: the best I got was sending a packet to the router directly but I couldn’t get my other machine to read it because my router was blocking my packets.

It was a tricky problem and wanted to know what you guys think about it. So yeah, what do you think?