If this persists, it would award an altogether new topic, since the issue you initially addressed seems to be solved.
Resolving it might be simple (or not), but will be highly specific to your router model (click for details).
Your observation implicits that only internal network names (like pi.hole) don't get resolved once you alter the router's DNS settings to use Pi-hole.
This might be explained by your router suppressing an upstream (supposedly external) DNS server's answer that points to an address in your local network. This is a security feature - normally, an upstream DNS server cannot know your internal IP addresses, so an answer like that is deemed as an attempt to breech into your network from the outside. Some router models allow to define exceptions for this DNS rebind protection, but your router might not.
Furthermore, this would only apply if you configured Pi-hole as upstream DNS server, as opposed to propagating Pi-hole as local DNS server. Local DNS is the preferred variant, read more about this in Why should Pi-hole be my only DNS server? and How do I configure my devices to use Pi-hole as their DNS server?
The vast majority of routers will allow customising upstream DNS servers (some may not even allow that, using just the DNS servers as supplied by your ISP). But not all routers offer local DNS server configuration, usually done over the LAN's DHCP settings, but sometimes hidden on the upstream page as an option like 'DNS relay'.
You can find out whether your router is merely using Pi-hole as upstream DNS server or propagating Pi-hole as local DNS server by executing the following command in a terminal window from a client (e.g. your iMac):
nslookup pi.hole
This will produce an output like:
Server: raspberrypi
Address: 192.168.1.31#53
Name: pi.hole
Address: 192.168.1.31
If Server and Address in the first two lines show your router's name or IP address, then your router is configured to use Pi-hole as upstream DNS server.
Whether your router allows setting up Pi-hole als local DNS server, or defining DNS rebind exceptions, and how you do that exactly, will depend on your router's make and model and even firmware version.
I am not familiar with Orbi routers, so I cannot help you any further.
So, the best option would be to open a new topic for this, with a title expressive enough to attract the right people's attention.