Well you're doing it in a weird way that seems to fix an annoying behaviour of some or maybe all Asus routers.
What I can read from your posts, it seems like your clients now have 2 path's for DNS resolution:
- [Client] --> [Pi-Hole] --> [Google DNS] --> [Root servers]
- [Client] --> [Asus] --> [Pi-Hole] --> [Google DNS] --> [Root servers]
Your not suppose to offer the router as an DNS resolver to the clients (only Pi-Hole).
But the annoying thing about my Asus router is that besides the DNS server configured in the Asus DHCP server settings, the router always pushes, via DHCP, its own DNS service too.
This means the clients will always receive the Asus routers IP address for DNS resolution plus the one configured in the DHCP server settings.
If you were to default your routers WAN DNS setting, you would have this situation:
- [Client] --> [Pi-Hole] --> [Google DNS] --> [Root servers]
- [Client] --> [Asus] --> [DNS from your modem] --> [ISP DNS] --> [Root servers]
That would mean that the second path wont resolve using Pi-Hole so about half of the time, ads will be displayed.
You just lucky you set it up this way without knowing about the Asus annoyance I have.
But most importantly, it works 
Keep in mind that your stats will be off now as about half of the requests processed by Pi-Hole will be coming from your router.
You could test responses from both DNS services (path's) with the nslookup command in a command prompt like so:
nslookup pi.hole 192.168.1.1
&
nslookup pi.hole 192.168.1.179
Both should resolve to the same IP address 192.168.1.179.