I’m running the Merlin fork of the Asus firmware, so maybe that adds this option, but in my DHCP settings there’s a switch called:

Advertise router’s IP in addition to user-specified DNS

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Sidenote: The Merlin version of the firmware is great! It lets you run the Diversion adblocker, which functions in the same way as Pihole. However, the RT-AX55U isn’t supported. The AX58U is though, maybe it might be worth upgrading? It’s a lot simpler than running a Pihole separately.