Two tier DNS? How to

I have just converted from ADSL to FTTP. I am hoping to use the provided Fibre Modem as a DMZ to my existing network router.
I think that that bit is working - I can ping (8.8.8.8) from my workstations.
However, I have a problem with DNS. The Fibre "knows" how to resolve address on the Internet but knows nothing of my network static address. While my original router knows all about my network but can't resolve internet address.
Can I set OpenWRT so that my network client can ask tOpenWRT for an address but if that in not known (it will be an Internet address), the OpenWRT DNSMasq will forward the request to the fibre router?
Regards, Martin

See https://openwrt.org/docs/guide-user/base-system/dhcp_configuration

Normally you just configure some domain as local and dnsmasq will resolve it locally, other domains will be forwarded to the upstream resolver.

How is the OpenWrt router configured an connected to your network?