The first thing is that you get yourself a suitable antenna. The antenna needs to cover the frequency band your provider is using and the antenna needs to have reasonable strength (called "gain"). To be flexible, ideally the antenna has a high gain across all possible frequency bands in question. Most of Ameison's mobile broadband antennas cover all relevant frequency bands from 860 - 2200 MHz, and therefore will suit any available provider. Often you can find some low price "high gain" antenna, only to discover that it does not work well for your case. Sometimes it is quite tricky to find out antenna gain over different frequency bands, because antenna companies who provide these cheap "high gain" antennas will not give this information. The only way to find out is a datasheet, which actually shows the gain of the antenna across the whole frequency band as a graph.
The second thing is to get the antenna outside or at least as close to the window as possible. This is extremely important. Walls eat signals. You may know this from your own experience. Stand inside a well - built house with lots of steel and concrete around you and you will notice that while talking on your mobile phone the connection gets worse while you move deeper into the building and better when standing next to the window (or hanging out of the window...)