Why can I connect to VPN on my iPhone on 4G only?

[Fzang]

I can connect my MacBook and iPad to VPN over wifi. No problems.

However, with my iPhone it only works on 4G. What gives? To make matters weirder; I can enable wifi and still keep my VPN connection.

 

[BrianBaughn]

Old thread but I've experienced the same thing this week minus the enabling wifi after connecting part. Haven't tried on an iPad but works fine on a MacBook Pro on OS X 10.11.2.

It's an L2TP VPN.