I had this same problem but Pihole can act as your DHCP server too. I turned off DHCP on my ISP router, turned it on in Pihole and configured my range (with some buffer for static IPs for servers and others) and off it went. When all my clients (laptops, workstations, phones, etc.) requested an IP (which I saw them trickle in almost immediately), they got their IP from Pihole and also automatically directed all DNS queries to Pihole. No need for complicated setups.