LTE modem external antenna

I am willing to install turris omnia with LTE at a place about 14km from the provider antenna.
This will be in Greece, so bands will include mainly Band 3 (1.8GHz), Band 20 (800MHz) and maybe in the future Band 7 (2.6GHz).
As I do not know anything regarding external LTE antennas, can anyone here propose a good external antenna?

First things first.

You should make sure that you have a clean or little obstructed line of sight to the tower.

Secondly, at 2.6GHz there is no chance over such a distance, so 800MHz or 1.8GHz are your only chance.
90% of on the country towers use 800MHz…but you should try to verify that.
The band is very important for the selection of a fitting antenna. While there are a lot of wide or multi band antennas their gain and characteristics for such a use case are less than ideal.
You should also try to minimize the length of the cable or consider using an amplifier or active antenna.

I have good experience with JARFT antennas, they promise 14dBi gain but come with a 10m long cable which eats a lot of the gain.
I trimmed it to 4m which resulted in about 4dB less in losses.