The best way to use a VPN with iOS is with IKEv2 directly in the iPhone settings and having Connect On Demand enabled. This is fast, leak-proof, and stable.
In other words, don't use an app. Just import the IKEv2 config file(s) for the server(s) you want. Apps are overrated, especially when you can get the TrackStop and NeuroRouting features to work with any device through the Config member area. Just my two cents after trying lots of crappy iOS apps.