I think I may have found a solution, I downloaded the update as normal then once download was complete I popped the sim out the top of the phone. Then I started the update to my phone. then when the phone restarted I reinserted the sim. Now when I restart my phone the WiFi calling reappears on its own. Now I know its a bit late since you've updated your phone but try this at the next update. I hope this helps my fingers are crossed for you.
