Let me add,
RoMon itself does not work over L3, BUT, if all devices are Mikrotik and running RoMon, you can access all devices over a L3 network via the RoMon agent.
If configured as per above, it will create a RoMon network, similar to what OSPF does for L3 routing
So if all OPs devices were Mikrotik, RoMon would have been a solution (Without the need for VPN - Not suggested though)