Hi there Rex,
Yup you can use your lte sim to your pocket wifi, You just have to create a new profile in your pocket wifi, See Below;
Connection Name: "You can put any name here)"
Username: Leave Blank
Password: Leave Blank
That's it your plan 1299 sim is now working on your pocket wifi, HOWEVER, you cannot bring your pocket wifi outside of the cell id your LTE sim was registered, in our case, it is the cell id near our home where our broadband was installed.
I too was searching on how to use this outside range of the cell site.