If you locate the PDF, it should include:

[Current Date] Subject: Analysis of ZTE F6640 device, mesh functionality, and guidance for obtaining the official manual (PDF)

You can download the ZTE F6640 manual PDF from the following sources:

on the F6640 until the LED blinks green, then do the same on the Agent device within one minute. Pairing is complete when the Agent's LED stays solid blue. ZLife Mobile App: Download the (available on Google Play