[inquisitor]

Thanks for pointing this out, Vlad. I wasn't aware of the refarming in Italy.

@Italia
While your existing antenna is compatible with the 900 MHz frequency band, your router does not support UMTS on this frequency. Especially in rural and costal areas it is very likely UMTS900 coverage to surpass UMTS2100 coverage because the lower frequency has a higher range and better signal propagation properties, which makes it more cost-efficient for operators to deploy it to less densly populated areas and such with unfavourable topography.
Even if your place isn't covered yet by UMTS900 you should in any case consider compatibility with the same when procuring hardware.
Those USB modems I recommended are all UMTS900 compatible in general but nevertheless check with the seller if the offered device does actually support UMTS900 and if it has an external antenna connector because Huawei produces countless subversions of their USB modems which sometimes lack an antenna connector or certain frequency bands.
For antennas it is a bit easier because the middle frequency of the UMTS900 band (920MHz) has a wavelength of 32.59cm while that of he UMTS2100 band (1920 MHz) has a wavelength of 15.61cm, which means they differ by a factor of 2.09 so any 900MHz antenna should work for 2100 MHz and vice versa even if sellers don't mention this explicitly.