Welcome to iMore! You don't need the cellular model to us Skype but you will need to be on a wifi network. Cellular cannot be added at a later time but you could always use a wireless hotspot. I use my iPhone 6s Plus as a personal hotspot all of the time. Just keep in mind it will use data so if you're on a limited data plan you'll want to be mindful of that as video calls like FaceTime and Skype use a lot.