Port forward issues with 2 computers

[Mr.Womburger]

The other day, I port forwarded so my friend can join me, and it worked immediately and flawlessly. The next day, we tried, but this time it was not working at all. Nothing changed between the days. Checking with this site gives me red, "Connection timeout (Resource temporarily unavailable)" all the time.
So now, I got the idea to port forward and host with a different pc. It worked flawlessly, friends can connect. It seems suddenly the game on my main PC refuses any online connection. I even manually checked firewall, it is set to be able to go through. I can't even connect the game to my secondary PC from my main PC. How can this be? There are no other net connection issues here, as far as I can tell.

 

[Panda.]

Check whether your IPv4 Address on your main computer got changed. If so... Rechange it too on your port

 

[SeventhSentinel]

Sounds like you haven't set a static IP. I don't have time to explain how to do this at the moment, but you should be able to find out how to set it via your computer's network settings if you do some Googling.