Isolate a pc with the router and download advanced ip scanner. It will list all active ips and there should only be 2

Are you using https?

Use https://x.x.x.x not http://

Then go to your server dhcp and put a dhcp exclusion for the pi hole ip and anything else you have static’ed

Plug a pc straight into the router and do an ipconfig /all. It will tell you the router gateway if you don’t have a static ip. Go to the gateway ip and turn off dhcp.

Are you on the same subnet as your router or are you on the subnet that your custom dhcp server is handing out? If your router is 192.168.1.1 and your ip is on the 192.168.2.x range, they aren’t going to be able to communicate.