Can anyone shed some light on how you are supposed to configure the LAN IP address on the Zyxel P-870H-51A V2 vDSL routers?

Whenever I try and change the router's LAN IP address I cannot get back into the web config.
Just keep getting the 'Page cannot be displayed' error when trying to access the new IP address.
Trying to access the original IP address also does not work.

The router does actually work on the new IP address, EG: I can ping it and it connects to the internet and traffic passes through it OK, it's just the web config isn't accessible any more.

I have a couple of these routers and they both do the same thing.

The only way to get back into them is to do a hard reset with the Reset button on the back.
Everything else about the routers works perfectly, it's just the problem accessing the web config pages once the IP address has been changed from the default.

They are both 2talk supplied so not sure if that makes any difference?