given that you have VLANs that should have an active DHCP server that provides the IP addresses to your clients, obviously this will directly give the client the IP address of the DNS server or the NTP server it should use
test this:
reference document:
https://openwrt.org/docs/guide-user/base-system/dhcp_configuration#dhcp_options
example on my guest interface
for ntp server, option 42
example on my iot interface
for dns server, option 6
