Listen Server Help

I have a listen server, i always am on it and this usually happens every so often, the problem is that when i play some days my friends can join and people who see my server can also, and some days they can’t join only see my server,i didn’t change anything also. It happened today when i was going to have a ton of my friends join but then they cant join. They told me that the server was not responding or failed after 4 tries.

Does anyone have suggestions or can help me?

And i cant make a dedicated because i have an AT&T router.