Remote Office, second unit via L2TP VPN or tailscale services? Using Active Directory only on the main office

I got a setup like this: Behind a Unifi setup, we have MS Active directory with all the users and only using it was a directory, everything else is Synology centric. Main NAS is connected to AD and everything is running fine. Now a new remote office is being setup and just got a second NAS for it, considering and trying to setup this second NAS to connect to the first one via VPN, but connecting to our infraestructure via UNIFI.

The nas can connect to vpn and can see the AD server, even talk to it, but not join it.

At this point, im gessing the issue is that the second unit connecting via vpn and the AD server thinks the nas is using the Router VPN tunnel IP. Now the problem is that the synology dialog box doesnt say much at all.

Anyone has any pointers/experience/suggestions with this type of setup?

are both Synology units on the same IP scope, but just at different sites? if so, you need to fix that first. 192.168.0.0/24 at one site, 192.168.1.0/24 at the other site. Else you have a IP conflict locally that needs to be resolved.

I should add the final goal

once the nas is up and always connected via vpn, the local users will use this second nas to access SynologyDrive and use active backup for business to backup their laptops. So SynologyDrive should sync all files between the units

Once that is finally working, a third NAS on another location will be used as a backup vault for all others.

Site-2-site VPN solution at the firewall level would be my preferred configuration….

Their scope is totally different. the second NAS is also getting an ip like 192.168.4.0/24 which is automatically assigned by the Unifi router DHCP vpn.

That would be optimal, but I dont have full access to the infrastructure at the new location (shared office). So I opted to get the nas to connect directly