So, if i understood correctly, your DNS server is running on your local LAN, and your need that the vlans IOT and GUEST be able to access your LAN, so their can reach your DNS.
Guess that what is missing, is allowing zone forwarding to your LAN:
-> Click on the pencil on IoT/Guest zones: https://prnt.sc/aCttZRpJj6gT
-> On "Allow foward to destination zones" add "LAN", should look like this: https://prnt.sc/4cU-u1UVw07P
-> Save and apply.
-> Do the same for the other zone.
If it doesnt work, please draw a simple topology with the VLANs IP's and devices. Also please attach a troubleshoot file.