Block Custom DNS Synology

[gp-se]

Hey guys, I have a Synology RT-2600ac running the latest 1.2 SRM firmware. In the firewall settings how can I disable all custom DNS servers that are hard coded on my IoT devices? I want all the connected devices to use the DNS resolver I specified in the router, not "custom/google dns" that is hard coded in some devices that are connected.

thanks for the help