SMS Relay – Hardware requirements? I am unable to activate SMS Relay (aka Text Message Forwarding) between my iPhone 5s running iOS 8.1 and (a) my iMac running Yosemite and (b) my iPad Air running iOS 8.1. Neither the iMac nor the iPad displays the activation code which is supposed to appear when Text Message Forwarding is activated on the iPhone.

The iMac is shown as "iMac (24-inch, Early 2009)" on the About This Mac summary, so is no doubt too early to support SMS Relay (elsewhere I've seen references to 2011 models being the earliest supported). So my first question is: [1] Am I out of luck even trying for SMS Relay on my iMac?

My iPad Air is only a few months old, which I would expect to meet the requirements for SMS relay. However, it is a Wi-Fi only model, so my second question is: [2] Is it necessary to have a cellular iPad for SMS relay to work?
I think you are confusing this with the requirement for Bluetooth LE to be able to do Handoff.

SMS Relay uses Wi-Fi or, if available, your phone's 3G/4G connection.

From How to Enable SMS Relay on Your Mac, iPad and iPhone - point 3 might be the salient one...


  
  Make sure your Mac, iPad and iPhone are all connected to the same Wi-Fi connection.  
  Make sure all of the devices are logged-in to the same iCloud account.
  You don’t want to be receiving your father’s phone calls, do you?  
  Starting with your iPhone first – and this is important, because I did it backwards and messed everything up – open Settings > Messages and tap “Text Message Forwarding.”  
  Choose the devices on your home network that you’d like to receive SMS messages on – in this case, I chose my MacBook Air and my iPad.  
  You should receive a code on your iMac and iPad.  
  Enter that code into the prompt on your iPhone.  
  


This might also be useful - FYI...SMS Forwarding does not require WiFi to work