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You can push DNS servers for DHCP clients with the help of DHCP options. Specifically for DNS servers, option 6.
Go to Network → LAN → DHCP Server → Advanced Settings. Look for the DHCP Options field. For instance, if you want to push 126.96.36.199 DNS server, you would have to enter the following:
More info here: https://wiki.teltonika.lt/index.php?title=LAN
More on DHCP options: https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml