If you are using branch office you will have routers between them, of course.
You haven’t told us how you are searching the V90s but when you are using “Find Devices” via subnets the routers will prevent WDM from sending UDP broadcasts out to other subnets then its own.
To guarantee a 100% communication you should add so called IP helper addresses on your routers. Normally each router is using IP helper addresses to make sure DHCP is working.
So if a unit in a subnet is sending out a DHCP request it will directly redirected to the DHCP server in a different subnet via the IP helper address. Now add a second IP helper address to your router and point it to the WDM server.
As long as the units in your branch offices did not have a WDM server configured locally already (as you have written something about migration this could be true), the units should find the WDM server automatically.
If not, search either via IP ranges or better enter the IP address of the WDM server in the Control Panel applet Rapport locally on the unit (don’t forget the Write Filter).