Cehan, In order to solve your problem, you need to understand path loss of signal levels may be beyond your expectations. This range of transmission is normally performed by 50K$ microwave towers with big Andrews Dishes.
I would start by finding a program ( in Linux or XP ) that can read your wireless modem adapter's Receiver chip using a signal called the Received Signal Strength Indicator (RSSI) in dB. (THe bars indicator in WIndows is not very useful) , I haven't seen any wireless Routers provide this info, ( unless you can find a Telnet solution to the Router's Linux firmware )
Without this data, you are unable to get a handle on the problem.
With this data, you can see signals down to -100dB but need -75dB for reliable 11Mbps communication.
WiMax is "a standards-based technology enabling the delivery of last mile wireless broadband.
It does not guarantee anyone can receive at 10 km.
Alternate solutions are a daisy chain of routers which can act as repeaters using RIP protocol from friendly people who are willing to participate. I saw this in practise in border cities on the North Island of New Zealand along the West Coast, so Wifi can reach all the beaches area and downtown small villages. Only MAC registered Routers could tap into the network using RIP protocol.
Another solution might be Ham Packet Radio which I know started in Canada with 64kbps modems around Ontario here over 20 years ago. I haven't looked into it since.
If you could give any RSSI dBi level at some distance, then tell us what you have tried, then a solution might be possible, but without this, you need local support.
If you expect any answers here, you need to provide a LOT more info on hardware, software, and test results.