Open the following submenu on your iPhone and navigate to it:
Settings --> Mobile network --> Data options --> Enable LTE
Here you can choose now:
- voice and data
- Data only
- Off - Off
To deactivate LTE for calls only, select "Data only". You will now receive calls via the UMTS or GSM network, but you can still surf the Internet via LTE. In this case, it's not that bad if the connection is temporarily disconnected.