Q: How do I activate my iPhone 5S without cellular?

Are you getting an error message on the screen? If so, what is it?

~Lyssa

I would see if anything here helps: http://support.apple.com/en-us/HT3406

The SIM in the phone either needs to be active or used with a previous plan. You might be able to just put your SIM from your 6 in the 5S to get this activation bit dealt with.

~Lyssa

They are interchangeable; just power down both phones before swapping the SIMs.

~Lyssa

Just put your SIM card into your iPhone 5s to activate it. It doesn't need any cellular data if you can connect it to a stable Wi-Fi or connect it to a PC on which iTunes installed. Here are the guide to activate an iPhone: Get help activating your iPhone - Apple Support

Good to hear you got it working

~LYssa

I am trying to do the same thing.  I have an iPhone 5S that I want to make an iPod for my son. 

I erased all content on the phone because I'm trying set up the "family sharing" --> thinking I could "activate" it from iTunes.  It still has the same AT&T sim card, and it successfully detects wireless, but when I connect to iTunes via my Mac, iTunes detects the phone but gives an error "Your request couldn't be processed."

Any advice?

Wait a while & try again - and if that doesn't help, you will be better off creating your own thread here so someone can help

~Lyssa

Great, thanks.  I'm a newbie on these forums.