Problem

The UMS is residing outside the network which contains your thin clients, so it cannot wake up your thin clients by Wake on LAN.

Goal

You want the UMS to wake up your thin clients from outside their network.

Solution

If you are using UMS version 5.02.100 or higher and thin clients running Linux version 5.09.100 or higher, you can make a thin client act as a proxy which sends the Wake on LAN packets on behalf of the UMS.