There is no official or widely recognized firmware version called "39link" for the NR5103E from Zyxel or major mobile carriers (like Three UK, which commonly uses this router).
Zyxel NR5103E Firmware Update
Updating the firmware for a Zyxel NR5103E (often supplied by ISPs like ) primarily occurs through automatic Firmware Over-The-Air (FOTA) updates rather than manual downloads. How to Update Firmware zyxel nr5103e firmware update 39link39