Static DHCP entries location

I do not use the router as DHCP/DNS server.

However, I want to have the hostnames in the Device List (checking what in online and the wifi signal strength). I do not want to enter entries in both locations (my Pihole as DHCP/DNS) and in the Tomato.

I did not find a file in the router filesystem that contains those entries so I can change it. Can someone give me some tips?

I expected that I can copy a similar file to the one in PiHole to the Tomato over SSH and keep them in sync