This video covers split DNS using Samba.
Not using samba? Craft Computing has created a solid video on setting that solution up using pi-hole open software: • You're running Pi-Hole wrong! Setting... (remember to turn off your routers DHCP to insure pi-hole is dedicated for the job.)
This video requires you have a Samba DNS server running on your Internal network. If you don't know what that is, you probably don't but doesn't mean you can't. In our case we are using a Samba DNS server but conceptually, this same solution will apply to BIND and other software based DNS solutions.