You are going to need to use a higher gain antenna. The problem is, you may not be allowed to. That is determined by your local regulations.
The ability to change the antenna is dependent on what version of the module you have. If this is the U.FL or RPSMA version, then it is simply a matter of using a higher gain antennas (at least 6db more in gain) in the same 2.4 GHz band.
As for using a 900 MHz with a 2.4 GHz product, it is not recommended. The antenna is tuned to a specific frequency range. Using an antenna that has been tuned for a different frequency can cause issues.