This is a post that may help out others with OpenVPN on the android client having a fallback of 8.8.8.8, which skips the pi-hole completely when using VPN enabled on a router. I had a weather app on my phone that was a test to see if pi-hole was working and an advertisement would periodically show up. Here's a page that shows how to make 8.8.8.8 a static route on many different routers that will block it and use pi-hole only:
https://support.overplay.net/hc/en-us/sections/115001085113-Static-Routes
Also, at the end of my automatically generated .ovpn file for import into the client, I added this line to use the pihole as DNS:
dhcp-option DNS <your pihole IP address>
Hopefully this is helpful for people if they have built-in VPNs in their routers.