Samsung Galaxy S6 (unlocked) Manual update 5.1.1. UK

Hi, I have a Samsung Galaxy S6 originally on EE and I had it unlocked to 3. I am unable to do an update as via WiFi and I am presented with the following message, "The operating system on your device has been modified in an unauthorised way. Try downloading software updates using Smart Switch on your PC or visit a customer service centre".

I have been told that the unlocking of the phone has resulted in me not being able to use smart switch and that the phone is no longer connected to the Samsung servers.

The build number is LRX22G.G920FXXU1AOCY

I am using Android 5.0.2. at the moment.