I have a wireless router serving multiple devices.  I'd like one of these devices to also serve as the DNS server, as I was able to do with the Archer C7.  However, when I do this with the AX90, it throws a fit:

I have the DNS server set to use a MAC address reservation at a fixed IP address in the AX90, so it will always (mostly) come up with the same address

I have set the AX90 to use this fixed IP address as the DNS server

However, as soon as click "save" in the DNS server setting, I get lots of "thinking" and the AX90 will eventually switch IP address subnets (i.e. from 192.168.1.* to 192.168.2.*, and reassigns all DHCP clients, and takes away the address reservation previously assigned to the DNS server).  I will then get an error saying it did so to avoid a conflict.

Apparently, the DNS server cannot live in the same subnet, and this is a new restriction, perhaps?  I seem to recall having this same setup working when I had an Archer C7, but perhaps I did something slightly different in that case?

For instance, an address reservation for the DNS server that is on a different subnet?