This last bit helped tremendously; pi.hole was not resolving because it was pointing to the IPv6 server. Turns out that OpenWRT router was broadcasting itself as the IPv6 DNS server; I believe this was a default setting.
It needs to be disabled in OpenWRT under the lan interface settings:
Interfaces > Lan > DHCP Server > IPv6 Settings
Disabled the setting and rebooted the router (just in case) and everything seems to work as expected.
Thank you for your help!