Can't change Airport Extreme IP to 192.168.8.1

I've just received a new router from my ISP, and I am unable to change its IP to the same IP my old one had, and what I have configured my network to use. So, I have decided to put my router back to modem mode only and resetup my Airport Extreme.

The default IP (after resetting it) was 10.0.16.1, but if I change that to be 192.168.8.1 (via DHCP settings) then I lose my Inteet coection.

I managed to change it to 10.0.8.1 successfully.

I read somewhere that it has to be 192.168.0.1, but I'm pretty sure I have done this before!