If you open the discover.log on your thin client, you will see the registering sequence is like:
1. Current Gateway
2. Backup Gateway
3. DHCP option
4. DNS
5. Boradcast
The DM agent will try above values one by one with prioriy, the NEXT value will only be used when none of the previous value is available.
So in your case, simply set DHCP option for that specific subnet. As long as the Agent in the subnet can find Server from DHCP option, it will not try DNS way.