If a client gets two or more DNS servers assigned that are not Pi-holed, you can imagine this breaking the ad blocking when the clients query the other DNS occasionally.
Me and the user from the other thread were not able to get ride of that second DNS IP coming from the Asus router.
Thats why we disabled the DHCP service on the router and activated Pi-hole's own DHCP service as a replacement:
EDIT: Dont forget to renew DHCP leases on the clients when switching DHCP!
Leases get renewed when disconnecting and reconnecting clients from network or reboot them.
Might also want to clear browser cache and if know how, clear OS DNS cache too.
Your screenshot looks like a Windows PC.
To clear OS DNS cache from a Windows command prompt:
ipconfig /flushdns