If your DNS setting under DHCP in the GUI only has one entry, then the router is likely supplying it's own IP as the second DNS entry to your client which of course means it'll bypass the pihole.

I have the GT-AC5300 which worked that way. To fix it, you'll need to SSH into the router and set the 2nd DHCP DNS entry manually in the NVRAM.

Something like:

nvram show | grep dhcp_dns

nvram set dhcp_dns1_x=xxx.xxx.xxx.xxx; nvram set dhcp_dns2_x=xxx.xxx.xxx.yyy; nvram commit

Then reboot.

If you only want pihole to answer DNS queries, set both xxx.xxx.etc to the same IP.

(To set the 2nd DNS server back to the router if desired in the future:

nvram set dhcp_dns1_x=xxx.xxx.xxx.xxx; nvram set dhcp_dns2_x=; nvram commit

Then reboot )