If you are lucky, your clients are just holding on to an older DHCP lease with outdated DNS server information. Dis- and reconnecting them to the the WiFi network should solve this. Some stubborn smartphones might need a restart as well.
If that doesn't fix it, you might be suffering from a strange misbehaviour on part of your acccess point:
Only a while ago, some ASUS network equipment (especially Asus AC68) were notorious for handing out themselves as DNS server in addition to any manually configured DNS server.
It seems some users were able to avoid this by filling every available slot with Pi-hole's address. However, there seems to be no guarantee that this will actually make the router disappear as DNS server.
As a safe guard, you could also configure Pi-hole as your router's upstream (WAN) DNS server, in addition to DHCP settings. On the downside, you should be aware that DNS requests arriving at Pi-hole in that way can no longer be associated with single clients.
Switching DHCP to Pi-hole would have been your best option, but:
That's weird, as having made Pi-hole your DHCP server should have dealt with the problem, handing out DHCP correctly. Did you disable DHCP on your ASUS device?
I also have no idea whether this behaviour is limited to certain router models and (custom, e.g. Merlin) firmware versions only, nor whether ASUS as the router's maker was made aware of this, whether a firmware update is available or even whether this problem still persists.
You could verify if your router is affected in this way by looking at cat /etc/resolv.conf on a Unix client or ipconfig /all | find "DNS"on a Windows machine. If your router's IP shows up aside (or even instead of) Pi-hole's, you are affected by this router problem.
In that case, it's probably better to search for a solution on forums dedicated to your router's version or custom software.