Very low rx power with Huawei SFP module

Hi there,
in a parish we have an internet contract with an italian ISP, Fastweb, who gave us their router with an external ONT (stick).
We put the ONT inside the SFP+ slot of Mikrotik (syncing at 1gigabit) but there is an issue in download speed, which is under 20 megabits (the upload is ok and it is around 1 gigabit).

In fact, looking at the SFP statistics from the sfp-sfpplus interface, I can see

  • TX power: 2.750 dBm
  • RX power: -18.416 dBm

As far as I know, -18dBm of rx power is a very bad signal..
If I plug the same GPON stick into the ISP router I get 1 gigabit on both download and upload.

I can’t understand what could be the reason for this issue.. anyone do?

Thanks in advance!