Some towns in Portugal have not 1800MHz network

[inquisitor]

That's normal. As far as operators have licenses for both frequencies, 900 and 1800 MHz (which is the case for all Portugese operators), they will always prefer 900 MHz over 1800 MHz, since the former has nearly double the range of 1800 MHz and so they need less base stations for the same coverage footprint. 1800 MHz is only used to expand capacity in urban areas or wherever necessary.
In other European countries there are operators, who have only 1800 MHz spectrum - there you would experience comprehensive coverage with your phone, as they don't have the alternative of going 900 MHz.