For such stuff I always recommend to make a drawing and incl further info like version numbers.
What do you mean by "no dual NIC". You do not have the option key or you have it but only use one interface?
If you try it without the dual interface option which is also needed to define the external IP
address it is not supported and no surprise that it fails.
If you have the dual nic option be aware that you have to define the external IP for that
interface and that signaling has to go to the external ip address and not the internal one.
VCS-C > SRC-NAT-VCS (optional) > EXTERNAL IP (VCS-E) > DST-NAT EXT> INT > INT VCS-E