You have probably done these basic checks already, but I will just mention them just incase...By the way I have never used PiHole so apologies if I mention specific resolutions...
This sounds like a DNS (Domain Name Server) issue..
If you are running PiHole as a DNS server, then you need to be googling for something like "Pihole and Dns not connecting to internet"
1) Can you can ping sites like yahoo.com from inside your network, using the Pi and any other computers
2) The Computer (not the Pi) can ping yahoo.com and if it can, check its DNS settings(on its ethernet card settings) to see if it has been manually set because this would mean that it and the router knows where to communicate with each other to resolve the yahoo.com address and connect to yahoo.com, but maybe the raspberry does not have a DNS set manually.
These are the sought of things I would check on the PiHole site by just searching google for something like this...
Google Search = "pihole setup for external connections" to see if others are discussing a similar problem.
I got this page
https://discourse.pi-hole.net/t/frustra ... ernet/8040
Google search "pihole how to set it up" - not because you don't know how to do it, but looking at different ways that people have gone about it will provide you with more experience as to where to look for an answer...
https://blog.sleeplessbeastie.eu/2018/0 ... e-pi-hole/