@Saku241 

Using the AX55 DHCP server, you can not have separate pools for assigning IP addresses.  It only allows one range as far as I know.

How is the AX55 connected to your ISP's modem/router?  If you are using double NAT and the AX55 is on the ISP's LAN, you could assign your DNS server an address on that LAN which then should be reachable by all of the devices on the AX55 network.

  @Saku241 

With the hardware you have, I can't think of a simple solution.  Something I tried here was to install another router downstream for the IoT devices.  That means double NAT and also opens up a possible security hole as the IoT devices can access the upstream router's network (where your DNS server would be).

For now the only simple solution I got is to set 192.168.1.2 as primary DNS and 1.1.1.1 as secondary DNS, it works but does that mean, every device on my network will randomly choose the DNS ? Or will it always prioritize the primary dns ?

  @Saku241 

The answer to your question depends on each device. For example, I have 2 IP telephones and I can set the dns queries to be in parallel or sequential. If sequential, one would assume the primary server is checked first.  The same is probably true for most devices.  Your solution should work fine even though there may be a very short delay when the primary server does not respond.

If you want to investigate further, you will probably need to use a packet tracer, such as Wireshark, to see the query order.