I dont know where you are based Barbar but 900Mhz is not a legal band in Europe or in US/ASIA as far as I know.
Also at 900Mhz the dominant propagation mode is line of sight,and at that range you need to consider your antenna/output stages carefully.
In my experiance you can expect a transmission on 433Mhz with an eirp of 10mwatts to give a range of perhaps a few Km with good conditions at low data rates but it built up areas maybe 0.5 KM at 9600bits per second