We had other users reporting their Netgear router's as being not very cooperative when it comes to customising DNS. 
As for a), it seems you currently have not configured Pi-hole as your upstream DNS server yet (or no longer?).
If your router supports it, b) would be the preferred way.
However, your router's LAN Setup screenshot doesn't seem to expose any DNS related settings to be distributed by DHCP.
As your screenshots look similar to those of another recent user, you probably should check No internet connection when setting DNS through Netgear N300 router Firmware V1.1.2.18 - #16 by devinberg and see if disabling your router's DHCP server and using Pi-hole as both DHCP and DNS server would work for you as well.