Whether DDC/CI works with CEDP is very hit and miss, even from unit to unit. Implementation is not as standardized as one could wish.
According to what Jack Bingham (CEO) said on their forum, Dell is a particularly hopeless moving target, because they source their parts from different vendors depending on which is cheapest that particular week.
I tried it on two monitors that were both supposedly supported in CEDP, one Eizo and one NEC. No way it would work. So I've given up on CEDP and use EasyPix and Spectraview II on these.