I had the same problem after installing Win 8.1 on my Lumia 521. On the home screen, scroll down to Wi-Fi Calling and turn off Wi-Fi calling but select Wi-Fi Preferred under Preferences. Next swipe left and go to settings and scroll down to Wi-Fi under system applications and select Wi-Fi then scroll down to Wi-Fi Sense. Tap Wi-Fi Sense and turn off both Connect to Wi-Fi hotspots and Share Wi-Fi networks. Go back to home screen and you should see a Wi-Fi icon at the top. If not, swipe left and go to settings and tap Wi-Fi networking. You should see your network listed, click connect and enter your network security key. Hope this helps, it worked for me and I no longer get REG90 error messages and can make and receive calls over Wi-Fi networks I select
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