Screen Sharing App Keeps Getting "Connection Failed to ..." Message

I’m trying to help a friend remotely over the Internet with their computer and keep getting an error message when trying to connect using the Screen Sharing App. Does anyone have any advice about how to get it to connect properly?

When I type in vnc://[IP address] I get the message:

Connection failed to “[IP address]”.

Unable to communicate with “[IP address]”. Make sure the remote computer is available and the firewall is not blocking screen sharing.

My friend’s system is running a desktop with OS X 10.5.8, connected via an Arris NVG448BQ modem.

I went through having them configure their Screen Sharing settings appropriately using the Apple Support article:

https://support.apple.com/guide/mac-help/turn-screen-sharing-on-or-off-mh11848/mac

We tried both their external IP (found by running Terminal and typing “curl ipecho.net/plain ; echo”) and their shorter local one (which was just “vnc://host-2.home/“) and neither worked.

My system is running OS X 10.14.6.