NameServers for a DNS Zone not working

Hello,

We have a Windows Server 2016 running AD, we added a new Zone to its DNS server and configure that Zone to have not only the internal NameServers, but also its external NameServers, but for some reason it only uses its internal NameServers, so when an external site is called, it can't find it, for example:

Internal URL:

https://intranet.example.com

External URL:

https://www.example.com

What can we do so it points to the correct destination?

We want the Windows DNS to look in both places Internal and External NameServers for this.

Thanks.