xize11
2
Could you sent a diagram how your network looks?
From what I think why it might happen is because your pihole is before the flint and not connected as client on flint?
Basicly it works as follows: everything on wan will see your router ip as client, so if you would have a setup like modem->flint between the modem and flint its being threated to use only the routers ip as client, and vise versa on the side of isp they will see your modem ip.
So in order to have clients work…
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You can do the easiest and connect the pihole on the router as client and make flint advertise this dns.
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you can also turn the flint router in a switch setup by moving the modem cable in ethernet 1 or turning wan to lan (edit: quick search says you can use Access Point Mode), you dont have to use dhcp anymore on the flint.
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or you may even be able to lease a section of flints dhcp and gain access, or by advertising the dhcp (it might not work, also things like authorative dhcp and fw zones come to mind which can make it difficult, and upstream is never aware of downstream).