Posting my solution in case anyone else is having issues getting their shares (smb/time machine) to be auto discoverable on OMV6 on the "wan" interface.
So I was having a problem getting my Time Machine share on PiNAS to show up automatically on my Mac after configuring it in OpenMediaVault 6 (OMV6). This service discover is handled by the Avahi Daemon Service.
"Avahi is a system which facilitates service discovery on a local network via the mDNS/DNS-SD protocol suite." -http://avahi.org/
In OMV6, you need to add an interface in the Network>Interfaces section for it to be included avahi configuration.
However, since the standard "wan" interface on the interceptor creates a "wan" VLAN for the first network adapter, there is no straight forward way to add it to the OMV6. If you try to add eth0 and then reboot, you will be in a world of hurt since swconfig (used by the OS) and netplan (used by OMV6) start fight over eth0 and the result is no network access.
The solution to this problem is to add "wan" to the "allow-interfaces: section of the avahi configuration file. This configuration file is managed by OMV6 so it will overwrite any changes you make. As such you need to follow the procedure to add this as an OMV6 environment variable via these instructions: https://docs.openmediavault.org/en/6.x/various/advset.html
Update the OMV_AVAHIDAEMON_ALLOW_INTERFACES environment variable to get OMV6 to include the "wan" VLAN as an allowed interface:
sudo omv-env set -- OMV_AVAHIDAEMON_ALLOW_INTERFACES "wan"sudo omv-salt stage run preparesudo omv-salt stage run deploy
I suggest rebooting after that procedure.
Once added as an environment variable, service discovery on my Mac started working for the Time Machine share.