A file is a compressed archive format (similar to .zip or .rar) that utilizes a high compression ratio. When you download tools to repair IMEI numbers—specifically for niche router models like the ZLT P21—modders and developers often package the necessary software into a .7z archive to save bandwidth and keep files organized.
Most "repair files" found online contain a generic QCN. If you flash
The IMEI serves as a unique digital fingerprint for every mobile device, allowing network providers to identify valid hardware on their towers. For a specialized device like the ZLT P21—a 4G LTE indoor CPE router—the IMEI is critical for authenticating the device with cellular service providers. When this number is lost or "null," the router cannot register on the network, rendering its internet broadcasting capabilities useless. The compressed file mentioned, usually a 7-Zip archive, provides the bridge between a non-functional brick and a working gateway.
For official support, it is always recommended to visit a certified service center to reload the stock operating system.
Your IMEI is like a social security number for your router. Protect it, back up your NV memory when the device is healthy, and never trust a random compressed file from a stranger on the internet.