Connected to Wi-Fi but shows 'no network connection'

I am connected to a wifi network but I get "no network connection" when I try to place a call in airplane mode. Wi-Fi calling is on, this is nothing to do with my provider (O2 - I asked them) or the ISP (It happens on ALL Wifis, home, work, everywhere). I have checked another thread here with this same question but there was no solution. Help!

[Re-Titled by Moderator]