If you have two identical F670L units, you can back up firmware via serial TTL or JTAG (advanced, requires soldering). Not recommended for normal users.
Finding an official download link can be tricky because ZTE typically distributes firmware directly to ISPs rather than end-users. 1. The Official ISP Route (Recommended) zte f670l latest firmware download
If your version is more than 12 months older than these, you need an update. If you have two identical F670L units, you