See my response to this question on the Apple discussions.

[https://discussions.apple.com/thread/5770041?tstart=0][1]


  [1]: https://discussions.apple.com/thread/5770041?tstart=0

Basically, use a static configuration for your WiFi Network, but leave the Router IP address blank.  The LTE icon will be displayed, and the mobile network used for all non-local traffic (ie, Internet) but you will still be connected to the WiFi network can can use all local LAN resources.