Problem about changing PLC IP address unwanted

Hi
I use from Delta PLC and I use from EN01 module for communication
I assign an IP address to it statically. But I have problem.this IP after a while changed unwanted and I do not know how it changed.
Anyone can help me?
My email is: [email protected]

bootp/dhcp

I have disabled DHCP and assign IP to it in form of static.
In normal situation it should not change automatically.