If there is a DHCP server on the network when the 750-843 boots up and BootP is running, it will get allocated a new IP address.
Two ways around this, don't have it connected to a network with a DHCP server or do as delae suggests and turn off BootP using the Ethernet settings tool.
It can be downloaded or it might be on the installation CD, hidden away somewhere.
You will almost certainly have to run the WAGO BootP Server program to reset the IP address to what you want/need. This should also be in the installation CD somewhere. DO THIS BEFORE DISABLING THE BootP service!!
Iain