Can't upgrade VMG8825-T50K router firmware

I'm using router model VMG8825-T50K that was bought from eBay.
I understand now that the router was supplied by an Italian Brodband ("TISCALI") company.|The router workd in my country, but I can't upgrade the firmware.
Though Zyxel prduct dowload page offer Firmware contain letters "ABRN" (V5.50(ABRN.0)b5_20190923), the official router page offers a firmware with a letters "ABOM" (5.50(ABOM.8.1)C0).

Is there a way that I can upgrade my router to the official firmware?
If not, where I can fnind the Italian company latest firmware for this router?

Thank you! 

Here is the product download page:

Consumer Product Help Center