L2TP Client: iOS
Since iOS 14, Apple mandate that L2TP connections support ipsec - ie, IPsec over L2TP: https://support.apple.com/en-gb/HT211840.
IPsec is is not a feature we support on our L2TP service and so won't work from iPhone or iPads.
(OSX will work though, with a manual tweak, see L2TP_Client:_OSX