Enter an Internal address into my Router

Hello All,

I have recently purchased a DGND3700 modem router which i have connected to my BT provider and everything works correctly and i have full internet connection.

I put my Active Directory server into the DHCP address so that it is assigned a static IP address, and I then change the DNS setting on the router under Basic Settings -> Domain Name Server (DNS) Address to point to this server so that all my computers pick up this as the primary DNS server so that machines will use it to authenticate on my network.

When my router reboots, it freaks out and switches the entire internal ip address range from 192.168.x.x to 172.16.x.x saying:

To avoid a conflict with your Internet Service Provider, your router's IP address has been updated to 172.16.0.1
You must now update the IP addresses in your router's configuration settings for each relevant service,such as port forwarding and IP address reservation.

is there any way to disable this so I can use my AD server on my internal network.

Thanks