Have to type "heartbeat" in console every 5 minutes if hosting a listen server

I’ve forwarded my ports and disabled firewall, but clients can only join if I type heartbeat in console.
After typing heartbeat server goes down on internet again in few minutes, so I have to always retype it.
It didn’t happen before.
Any ideas how to fix it? Thanks.

If it didn’t do it before are you sure you aren’t having any internet problems?