Hello,
Once the DNS server is defined in Network -> Interfaces -> [interface general settings], I would suggest to add a firewall rule, redirecting DNS traffic from standard port 53 to 553. The idea is to replace destination port in the packet's header destined to DNS port 53 with a value of 553. For this you would need to login to the router via SSH and define the rule in the /etc.config/firewall configuration file. The rule should be similar to the one below:
config redirect '<n>'
option proto 'tcp udp'
option name 'DNS_FWD'
option target 'DNAT'
option enabled '1'
option src_dport '53'
option src 'lan'
option dest_port '553'
You may need to replace <n> with an integer value, depending on the order of the rules in the file.
You can also configure the rule or add specific details in the WebUI, Network -> Firewall -> Port forwards page.
Best regards,