Hi,
It should be on the left-hand side menu bar, under Network → Mobile:

Then turn on the Enable automatic switching slider to see the rest of the options.
If it's not there in your case, please try to reset the device. And if that does not help, please send me a Troubleshoot file via private message. It can be downloaded from the router's WebUI, System → Administration → Troubleshoot page.
The WebUI layout is different from our older devices and there may have been changes from firmware to firmware as well. Perhaps it has become hard to find. :)
DM