Android wifi (oreo) dropping. Because of pihole?
I have a similar if not the same problem - my S7 (Android 8.0.0) works fine with the exact same setup that you have until "the next morning": I have to disconnect and then reconnect to my WiFi in order to access the internet (or get proper DNS resolution?).

I could not yet find out the root cause (is the DNS changed/lost?), but I just installed a network analyzer (called "Ping &amp; Net") in order to see if the DNS server on my phone is lost over night.