Check out the Ubiquiti Wifi antennas [1] for 25km+ connections using 2.4 and 5 Ghz. However, it is recommended to use the 5Ghz, especially if you have line of sight. What you need to find is someone on the other end who can put up an antenna as well and you can have a ~50MBit (depends on a lot of factors, this is an average based on experience if Barcelona and Guifi.net) connection for around $150.

[1] https://www.ubnt.com/