Necro-bumping this because I had the same problem as robikul and figured out what the problem was. Hopefully my issue was general enough that it applies to other people.
Same as that user, I have a Bell Home Hub 3000, and just as them changing the DNS server alone was insufficient, requiring disabling DHCP on the router and enabling it likewise in the Pi Hole software.
Similar to them, the device running Pi-hole (in my case a raspberry pi 4) then would fail to connect to the internet and get repeatedly disconnected from the network.
Despite setting up my raspberry pi to have a static IP on my local network through the Bell Home 3000 router, the missing step was to then in the DHCP settings in Pi-hole to likewise define an entry for the raspberry pi itself, entering the same IP and mac address and hostname. Once this was complete, restarting the Pi-hole and the router, everything worked like a charm.