Do I need a feed network when used two port (coax probe) for Circular polarization?
Do you specifically want the antenna to only act as one specific circular polarization? Indeed, CST likely only uses one port as default.
I am not familiar with CST myself, but in HFSS and EMPro you can select which ports you want driven and what their phase relationship and amplitude ratios are. This way you could make a circular polarization by driving one port with 90 degrees phase shift.