returned

Then I'd recommend turning off the DHCP server on the Asus router and enabling it on the PiHole... inside the PiHole DHCP server configuration set your routers IP as a gateway address, and set the PiHole address as the DNS. This way, PiHole will manage the IP addresses in your network, which might be a better option overall.

I'm an advanced Mikrotik user overall, one thing they had not been great about is WiFi but their networking capabilities have always been amazing but had a steep learning curve if you'd be using their more advanced options. Though Mikrotik has finally released their WiFi6 devices whose WiFi is finally getting competitive with other WiFi router manufacturers. I can recommend hAP AX2 or hAP AX3, and definitely check their Youtube channel for getting their tutorials.

Mikrotik is definitely my go-to recommendation if you also want to learn networking fundamentals because it's affordable and yet incredibly advanced whichever model you get because they all have pretty much the same RouterOS software within them. There's even more things to thinker about ever since the release of their RouterOS 7 which is now based on Linux 5 kernels compared to their old RouterOS 6 which was based on Linux 3 kernels.

You can even run PiHole on the Mikrotik router itself

Some devices get confused if there is only one DNS server provided by the DHCP... Maybe try setting your PiHole IP for both "Primary" and "Alternative" DNS server

The DHCP server pushes the DNS configuration to the clients. Is your Asus router running the DHCP server? If so, in DHCP configuration, set the DNS to point to your Pi-Hole

Yeah, I immediately thought of that too, but that's a relationship of an aunt and her nephew