Server not responding, but only when my partner tries to join me ??? [SOLVED: at least for now we think!!]

Hello! My boyfriend and I have been getting back into DST lately and playing every night, but one night he just couldn't connect to the server I was hosting
It said, "Server not responding. This server is not responding. Please try again."
He can't join any game that I'm hosting, and I can't join any game that he's hosting either.

Here's what we tried:
- We both disabled our mods and tried making a server with no caves
- Tried joining a public server together (which allowed us both to connect, but we still can't join each others games)
- Restarting our computers
- Restarting Steam
- Allowing access through the firewall
- Uninstall and reinstall
- Unfriending and refriending on Steam
- Tried a local server
- Tried a public server but with a password, which didn't allow us to connect if one of us was hosting :((((

We don't know what to do anymore, we just really like playing DST ;^;

EDIT: 
My boyfriend tried joining my game through steam and it still didn't work. But there's a page that says
"Failure. Unable to connect to server, app id specified by server is invalid."
"Latency. Failed"

I'm not too sure what this means, but I thought I should mention it if it helps! ;o;

EDIT #2: 

We've been trying and trying to connect, but today it worked all of a sudden? I think the issue somehow solved itself??

We think that maybe it was an administrative issue with the route or wifi? Something that we would have a problem with, but not necessarily fix ourselves.

I hope if anyone has these issues in the future that there is a concrete solution though!!