Yes if there is no option to set DNS servers in the LAN DHCP section of the router.
EDIT: Oh ps, I would change the order of enabling and disabling DHCP services!
First configure Pi-hole DHCP and enable it.
Check with below:
sudo pihole-FTL dhcp-discover
If that looks all dandy, disable the router DHCP service.
This because some routers re-initialise their network interfaces when changing network settings causing the clients to try and get a new DHCP lease while there is no DHCP service active yet.
And thus loose their IP details.
Including the PC from which you are changing settings.
To apply the new setup, your clients would need to renew their DHCP lease to receive the new DNS server (Pi-hole).
You can do that all at once by power cycling your router, switches and AP's.
When the clients detect network is restored, they will apply for a new DHCP lease and get one from Pi-hole including the DNS server.
In the end, only one DHCP service should be active on your LAN!