I can only help for the Italian language issue, but it is - I believe - about a different problem, a certain Italian ISP provides a Zyxel router that is somehow mis-configured and doesn’t reach the expected upload speed.
It is a peculiar case where the contract is about up to 2.5 Gb in download (but the actual infrastructure - exception made for a few rare cases/locations - tops anyway at 1 Gb in download) and 0.5 Gb in upload, but the upload is capped (by a bug in the firmware/software) to 0.2 Gb or less.
The thread is about downgrading (with the risk of bricking the router) to an earlier release that is capped to 1.0 Gb in download, but allows 0.5 Gb in upload.
Nothing to do with frequent disconnections.
This said, it is rather common that various manufacturers SFP’s have compatibility issues with Mikrotiks, but the particular model you have (PMG3000-D20B) seems to be working fine for other members, at least on other Mikrotik hardware:
http://forum.mikrotik.com/t/help-with-zyxel-sfp-gpon-module/175838/1