I’ve had the same issue with iOS devices using the Pi-hole as DNS (I.e. not set at router level).
Everything was fine for a few days but then it stopped - no web site (with or without ads) was returned. A reset (both from the web app and from the command line) did not help but a full power off and a wait of a couple of days (OK, I got distracted) seems to have sorted it.
I did change the DNS server back to the router and then back to the Pi hole again during this time.
Need Pi spray - these things are breeding in my house...