So I've ditched the database on the PC, and started from a reformatted Mac mini with OS 10.12.6. Now I am having no issues connecting machines to a shared database (Mac or PC).
I also updated Resolve to version 15.1.0025 before creating the new database.
Back to the PC though, I was able to Ping the port range recommended. So it all appeared to be fine. Now that it's working with a Mac, I don't want to look back!

I was only using the PC as a testing system before dedicating a machine to the database.