If you plan to stay for less than a year than real estate agents are no help anyway. The only choice is offering hosts a deal on a site like AirBnB. And there you deal with hosts directly who can give you factual information about their internet connection and 3/4 G availability there.
I've traveling and living for several months in different parts of Spain in last few years and 3G always was a good backup plan and was more than enough for trading. Having some sort of broadband contributed more to online movie watching than to trading :)