The term “Hard-coded DNS Clients” refers to clients which always wrongly-code a DNS request, sending it to a hard coded value they know instead of a DHCP recommended value. However some client make both rightly and wrongly addressed DNS requests, so here the term “Wrongly-addressed DNS requests” is used instead.
There are a number of discussions and tutorials on this topic on Reddit, UBNT community, and elsewhere. The usual solution is to implement a DNAT/SNAT pair of rules to deflect the DNS packet from the undesired external DNS server (e.g. 188.8.131.52) to the desired pihole DNS server.
The problem with that usual solution is that, under certain circumstances, the wrongly addressed requests are conglomerated and recorded by pihole as coming from a single single address, the gateway, e.g. 192.168.1.1. Using the UBNT ERLite 3 router is one such circumstance.
This post discusses a configuration to let the pihole know the seperate identity of each wrong-addressed DNS client, so each such clients statistics can be displayed seperately. In addition, for each client the rightly-addressed and wrongly-addressed DNS queries are counted seperately.