Problem: The IP address conflicts with the WAN IP subnet.

Problem:  "The IP Address conflicts with the WAN IP subnet.  Please enter a different IP Address.  NetGear wireless router DHCP range."

Symptoms:
On a NetGear Wireless Router, while trying to change the DHCP address range.

Left-Nav: "LAN Setup" panel, scroll-down
Starting and Ending IP Address

Issue:
On a NetGear Wireless Router, LAN Setup, Advanced Setup, trying to set a smaller IP Address range, changing from [192.168.200.2--200.255] to a smaller range, such as [192.168.200.10--200.100]

The error: Another hard-coded IP address is already using a number within the range [2-255] and you can't change this value, even if you are tightening it up.  It is unfortunate that NetGear selected such a broad default address range.  Existing devices can be seen in the router's "Connected Devices" report. 

I had this problem while replacing an older router.  In my case, this was a Raspberry-Pi web-server and a DNS-SinkHole, both occupying addresses within the original, expansive range. 

It is complaining about the existing range [200.2 -- 200.255]. 
Unclear why it whines about this now.

Solution:
 
Disconnect those devices from the network by either powering them down, or by unplugging their network cable (assuming hard-wired).  Cycle the router's power, re-login to "admin", and attempt to set the smaller range again.

Related article:
Installing NetGear AX7800 Wireless Router