Looking around, I subscribed a new contract with PLUS Mobile operator that was offering an unlimited data plan over LTE connection (PLUS LTE bez limitu).
When I checked the coverage signal map at PLUS website, my home was not covered by LTE signal, but I was wondering if using an high-gain external antenna could help me to get a stable and fast internet connection over mobile LTE and I decided to try.
So, I bought a 17db dual LTE panel antenna and I subscribed the new contract with PLUS. They gave me an LTE router (the Huawei B593s-22) that has two external SMA connectors to connect the external antenna.
To correctly point your external antenna you can use your smartphone and the Open Signal app for Android or iOS. You can check the operator's antennas even on Open Signal website.
Here the speedtest.net results:
Without high gain antenna
Using high gain antenna