I am having problems with maintaining a stable connection to my NAS drive caused by the dynamic IP address changing - I would therefore like to use a static IP address in the modem / router, but when I try to set one up I keep getting an error message, so obviously I'm getting something wrong.  I have tried using the current IP address of the NAS drive but am not sure that subnet mask and gateway should be used and it is probably one or other of these that are not correct. 

I have tried using: 255.255.255.0 for the subnet mask as this is what is shown for the LAN under 'Advanced Status'.  However, I don't know what I should use for the gateway - I get an error message if I leave this blank, so something is clearly required - I tried using 192.168.1.1 (the default gateway shown under 'Advanced Network' for the DHCP server) with Interface set to LAN, but this gives the error messge: ' Error code: 5105 Destination IP Address cannot be on the same subnet mask with the LAN IP. Please input another one.' 

If I leave it on the default WAN setting I get a different error message: 'Error code: 5105 Destination IP Address cannot be on the same subnet with the LAN IP. Please input another one.'

I have owned various different models of modem / router over the years and I've never had this amount of difficulty setting a static IP address - they have simply either given me of changing an existing dynamic address to a static by selecting a dynamic address under the client list or by simply requesting input of an IP address.

Any guidance on what the subnet mask and gateway should be would be appreciated