Netgear M4100-D12G - IPV4 Net Interface

Hi, please can I bump in here? I cannot follow the above but I am having real problems setting a fixed IP for management purposes. The default setting for the management interface is to have DHCP enabled and this is working fine. My problem is that I want to turn off the DHCP and assign a fixed IP. ie from addresses not available on DHCP but on the same subnet. If I turn off the network configuration protocol I am then able to edit the IP address but whatever I do I cannot make it stick. As soon as I apply the address I lose access to the device and cannot then save the configuration. If I try and access the web interface at this point I cannot access using either the new "applied" IP address or the original one issued by the DHCP server. How may I set a fixed IP for the management address of the device please?