I can connect to public wifi eg at the airport but then cant access the internet. WHy? Fix?

Use captive Wi-Fi networks on your iPhone, iPad, or iPod touch - Apple Support

After you connect go to settings/Wi-Fi, tap on the i next to the network name, then Forget this network. Then connect again and you should get the network sign-in screen.

The problem isn’t that the network is not secure, it’s that it requires a sign-in screen, and some public sites don’t reset correctly if you reconnect to them after previously connecting. MoorD10 posted a link to a good explanation of how it works (or should work).

The network is not secure if it is public. No fix for that. Look for a network that requires a Wi-Fi password to connect.

No fix for WiFi unless Apple decides to make things less secure.

Try using the cell service.

If only......but worth a try.