When your Pi-hole doesn't act as a DHCP server (because you already have another one) you'll able to see only the IP addresses of the devices.
Not really user-friendly when you need to analyze the log of the Pi-Hole.
But don't be afraid, there is a solution!
To be able to see the device name, you just need to enable the Use Conditional Forwarding setting in the Settings - DNS.
When the option is enabled, enter the local network to use and the IP address of your DHCP server as described in the screenshot, save and it's done. It's magic.