I’ve fixed it with some help from Avast. For anyone else experiencing this, here’s what you need to do:
"First please allow the Roon application in the Avast Firewall application rules: https://www.avast.com/faq.php?article=AVKB25
(set the program to be “always allowed”)
After that please go to the Avast menu - Protection - Firewall - Settings - PREFERENCES and check the “Internet Connection Sharing mode” option.
In the same window, go to the “System rules…” and enable the first two options (Windows File & Printer Sharing and Remote desktop connections).
Finally please restart your computer and try to play the Roon program again."
[I’d done the first but not the second of the suggestions above]
Thanks to @Eric for his patience and help.