...
At least one of the 'workstation' or 'http' or 'https' services has to be defined in the 'Avahi' for this local datastore feature to work.
Verify the App Side mDNS advertisement
An easy way to see if the App is running 'avahi-daemon' and advertising service/hostname correctly, One can issue "ps aux | grep 'avahi-daemon' " to see if it is running. Or one can issue "sudo systemctl status avahi-daemon" to see the 'avahi' status. Then install the 'avahi-browse' program by "sudo apt install avahi-utils". The 'avahi-browse -a" command output should display the hostname, service, etc info:
ubuntu@ubuntu:/etc/avahi$ avahi-browse -a | grep server
+ eth0 IPv4 ubuntu-4321-http-server [00:16:3e:00:01:03] Workstation local
+ lo IPv4 ubuntu-4321-http-server [00:00:00:00:00:00] Workstation local
Local DataStore Protocol Scheme
...