I can only guess here, since I am not at all familiar with that router.
It doesn't seem like you have IPv6 support in your network - I'd assume that your ISP would assign a prefix for you to use, but the only entry hinting at this in your screenshot reads 'Not Available', and it may or may not relate to your ISP's assignment.
Would you be able to confirm through other sources that your ISP provides IPv6 connectivity at all?
Another way your clients could bypass Pi-hole would be if they had access to mobile network services. At least your Android smartphones could have access to that.
Verify that they have mobile data access switched off.
You also described that you have verified that your WiFi devices have been correctly assigned Pi-hole as DNS server.
Let's check whether your devices do honor this:
From your Linux laptop, what is the output of
nslookup flurry.com