Why won't my Galaxy S4 connect to a Mobile Network?

A

AC Question

Galaxy S4 won't connect to a Mobile Network

My wife and I just switched to SIM-only contracts and picked up a couple of unlocked Galaxy S4's on eBay. We're in the UK; I'm on Virgin, she's on TalkTalk. My phone works perfectly.

My wife's, on the other hand, is unable to connect to the mobile network. We've tried her sim on my phone and my sim on hers, and the problem is definitely with the handset. it can see available networks, including hers, but when selected it just says "Unable to Connect. Try Later." This is different from selecting other networks which gives "Your SIM Card cannot access this network", or something to that effect.

Another oddity, which may be related, is that my phone automatically updated itself to the latest version of Android (5.0.1 as I write this), whereas my wife's is insisting no update is required despite running 4.4.2.

I've run through all the standard solutions available online and spoken to both phone companies and am kind of at a loss. Can anyone suggest what the problem might be? I'm happy to provide any additional information required.

Regards,

Dave