There is no reliable concept of primary and secondary, despite the labels. Given multiple
DNS Servers, clients are free to use any of them at any time.
With some clients and routers, in the absence of two entries in the "primary" and "secondary" boxes, the client or ISP will fill in the second silently. To avoid this, put an entry in each spot, and make sure each entry is a Pi-hole or leads nowhere.
Assume Pi-hole IP is 192.168.0.150
Try to enter this twice. Some routers don't allow duplicate entries, some do.
If you cannot enter this twice, put an unused IP from your LAN range in the second spot.
Pi-hole also has the option of being your network DHCP server. This has some advantages as described here: