Is it possible that you use a static IP on your RPi and the DNS is set to pihole? In this case your RPi does not use the DNS set in your router.
If this is the case:
Please change the DNS on your pihole device, not on your router.
e.g. your RPi uses also pihole as DNS and pihole does not work for some reason, you can’t reach the internet and you are not able to fix this, bc you can’t reach the internet 
Go into dietpi-config → option 7 Network Options: Adapters, then choose your network interface and change the DNS to 1.1.1.1 or whatever you like.
If the RPi use DHCP then something else might be going on.