"ZXHN F6640" filetype:pdf user manual
For more information on ZTE F660 and F6640, check out the following resources:
Here is the workflow the manual doesn't spell out clearly. You want to use the F6640 as your main router, but connect other ZTE (or even third-party) extenders via Mesh.
One USB 3.0 port and one USB 2.0 port for advanced content sharing. Advanced Features:
Power on the secondary node and wait for the light to stabilize.