It's an iOS limitation by Apple. Have you seen other VPN Clients with this feature?
At this momment, you need use the "Cisco" embeeded VPN IPSec client in iOS.
Cisco AnyConnect, OpenVPN and a Couple other iOS Applications have Tunnel Mode ability. Cisco AnyConnect has had it for a couple years already. Last I heard was Fortinet has sent in a request for enabling Tunnel Mode.
The new iOS SSLVPN Tunnel mode feature is currently under development. Current ETA is Q1 2016.
Technical Videos - video.fortinet.comTechnical Docs - docs.fortinet.com
Is this still on track for this quarter?
current ETA is mid-april.
Is there any pre-release notes/draft doc or mantis that we can reference?