Iphone not appearing in network options for tethering

My Iphone 6 which I use on a regular basis to tether my Macbook pro to my 3/4g is no longer tethering. The option to connect to the Iphone data via 3/4g has dissapeared from my network options menu. Al I have on the left to pick from is, Wifi, Ethernet, Bluetooth and Firewire.

How can I add my Iphone back onto this list? I can connect successfully with the wifi option and the bluetooth, but I would like to use the USB option, as I find it more reliable.

Thanks