Managing remote APs from Central UTM

I have a client trying to manage all their office APs from their Data Center UTM.

There is a VPN between the office and DataCenter.
We've tried setting up a static route for 1.2.3.4 from the Office LAN to the UTM LAN IP. The APs briefly were viewable on the the UTM, but wouldn't take any configuration (haven't been able to get this to work at all since).

We've tried adding 1.2.3.4 as an additional IP on the LAN interface of the Data Center UTM and updated the VPN info to reflect this.
(We can ping 1.2.3.4 and get replies, not sure how great a test that is though...)
The APs don't show up on the UTM and we don't see anything in the Logs either.

Is this possible?
What am I missing?

Thanks!!!