sorry I am not familiar with the Beckhoff programming.
However with most E/net or, E/CAT in this case, you may need to either save or apply the IP address change.
there should be a config area somwhere
First of all, the CX1100 is not a PLC, it is a part number for the power supply of a CX10x0.
Please check http://www.youtube.com/watch?v=8T171NIyH74, it is a video on how to connect to a CX PLC. I didn't watch it, but I guess it will describe how to make a connection from system manager, and changing the IP address. If not, I hope it gives enough background to google the solution.
If it still doesn't work, I have a CX1020 in front of me, I can make some checks.