Hello, I actually had a similar problem, but in my case, I wanted to use a specific DNS server. I solved this by modifying the /etc/dnsmasq.conf and adding the line "server= [DNS Server]"
I found this by reading up on OpenWrt, more specifically, Here
Good Luck, I hope this helps!
Here is my previous post
OpenWrt Documentation: https://wiki.openwrt.org/doc/howto/dhcp.dnsmasq