Once you have set the IP address of the Device via BOOTP you need to ensure your computer is on the same subnet to disable the DHCP other wise it cannot connect to do this.
make sure you can ping the device from command prompt before you attempt to disable DHCP, if it pings successfully you should be good to go.
the trap is once you use BOOTP to enter the IP it changes straight away and if your computer is not in same ip range it cannot communicate, hence why you need to change computer adapter settings, just remember to take note of what they are so you can revert back.
if the IP address you set the device is on same subnet you don't need to change you adapter settings at all.