Thanks for the replies,
I fixed my problem, I have now bridged my modem so all router capabilities are disabled and now my Asus router is running properly and the pi-hole works!
I've been trying to set up a Pi-hole but my Asus RT-AC1200 V2 router is not using the Pi-hole as their DNS server.
It seems that Asus routers are a bit tricky and I got the recommendation to disable IPv6 and use the Pi-hole as DHCP server.
The problem is that since a recent firmware update my...