https://www.asus.com/support/FAQ/1011703/
Enter under the DNS server under this page
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You need to find DHCP options under your LAN settings
DHCP is the service that provides information like the IP address, gateway, dns, and subnet to each of the clients in your network
In this case you want the DHCP server on your router to hand out your Adguard IP address as the DNS
I would also like it so if my Raspberry Pi is ever down it would just pick up a like normal just obviously with ads.
This is why it has fields for DNS 1 and 2. DNS 1 will be the primary, but if it's not working, devices will fallback to using DNS 2.
If you want to be sure devices will continue to work fine even if your pi is down, put your adguard IP in DNS 1 and then a normal DNS server such as 1.1.1.1 in DNS 2
After you change those settings, open a terminal on your PC and run
ipconfig /release && ipconfig /renew && ipconfig /all
scroll up to find your Ethernet adapter and see if the correct DNS server(s) are listed
there isnt fallback like that on dns.