This article speaks about how to perform speed test for LTE or any WAN interface from MIST GUI

From the Juniper Mist™ portal, you can run a speed test for a Session Smart Router deployed as a WAN Edge on your network. 

To run a Speed Test for a Session Smart Router deployed as a WAN Edge:

1. Navigate to the Juniper Mist portal, then click WAN Edges > WAN Edges.
2. Choose the WAN Edge device from the list.
3. Once the page is loaded, choose the LTE icon from the Port Panel.
4. The Networks section then appears. From the Networks section, under the Speed Test column, click "Run Test".

In the Speed Tests section, the test you are currently running is In Progress. After a minute or two, the progress of the speed test changes to Succeeded, and the results populate in the columns, displaying information such as download speed, upload speed, latency, the interface the test was run on etc.

The above steps can be followed for LTE and WAN interfaces as well.