DNS issue - Windows DNS & Cloudflare DNS

[wardv]

I have a problem with OPNsense, inside OPNsense I've specified the IP address of the DNS server, this is a DNS client on a Windows AD DC. In the resolv.conf on my OPNsense, it tells me this

domain wardverduyn.be
nameserver 10.0.0.5
search wardverduyn.be

If I now try to ping a host that is not in the AD DC local DNS, but in the DNS from Cloudflare on my domain, for example the A-record "mail.wardverduyn.be" is not inside the AD DC DNS, but it is in my Cloudflare DNS from the domain wardverduyn.be, this does not resolve. It only resolves records inside my AD DC or external records without the "wardverduyn.be". How can I fix this?