I just had a quick look at the manual for your router.
From the looks of it, the DIR-826L does not allow you to specify a local DNS resolver. In the WAN settings, you can specify an upstream provides, but if you put something on your local network it will not work.
The only (limited) options to change the LAN DNS seem to be on the "Router Settings" tab, and your choice is "Enable DNS Relay", which feeds your upstream provider to your hosts, or if it is disabled, the router serves the DNS requests.
So it seems you'll need to use your Pi-hole as a DHCP server if you want to use it as your network's DNS, which should be more successful now you've upgraded to 3.3.1.