The Zyxel NR5103E is a high-performance 5G router, but its firmware update process is unique. To ensure your device remains secure and operates at its best, be sure to log into your router's interface every few months to check for online updates, and if possible, keep the feature enabled in the router settings to ensure you receive updates automatically as your ISP rolls them out.
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I searched for information regarding a firmware update labeled "39link" for the 5G router.
The Zyxel NR5103E usually updates on its own when the ISP pushes new firmware. Ensure your router is connected to the internet. Leave it powered on overnight.
ISPs remove old firmware versions to force updates via their auto-update server. Fix: Do not search for archived links. Instead, enable Auto Firmware Upgrade in the router settings (Admin > Management > Auto Upgrade). The router will phone home to the correct internal link.