I dig up this conversation because I had the same issue with a newly installed pi-hole.
I turn off all devices at night, included the pi. So the pi-hole need to restart correctly each day. After one week trying, I finally succeeded! It’s a dirty solution but it work, maybe some of you will find something more clean.
For me, pi-hole can’t work as a DNS server when restarted because he don’t have the right time.
1/ I installed ntp on the Pi.
2/ I forced “nameserver 188.8.131.52” on /etc/resolv.conf
3/ I change the right of this file to 555 to avoid system modification each time.
After each start, the pi need a bit of time to sync with ntp server but after that, all good to go.