First, confirm that your router supports UPnP (also called NAT-PMP or simply port mapping). Refer to your hardware documentation to learn how to activate this feature. If UPnP is not available on your router, you must perform a manual configuration.
If you have more than one router on the same network, confirm that one of them is set to Bridge Mode and that all of them support UPnP. (Note that some modems also act as routers.)
Some manufacturers advertise their hardware as UPnP- or NAT-PMP-compatible, but some routers use buggy UPnP implementations. If you seem to be experiencing issues with your router's automatic configuration, try mapping the port manually.
Try restarting your router and computer. If this doesn't seem to help, it's possible that your router doesn't support UPnP. If this is the case, you must map the port manually or use another method (other than Screens Connect) to make your Mac remotely accessible.