Not the cheapest but a very powerful solution is to use 2 Ubiquiti’s NanoBridge M Model NB-5G22.
They have a small dish, about 14" across and models are available for 900MHz, 2.4GHz, 3GHz, 3.65GHz, and 5GHz.
They are WISP (Wireless Internet Service Provider) class devices that can use the AirMax (MIMO TDMA Protocol) to go over 30km distance at full power over open terrain. Overkill? Not really. The power output is adjustable using the GUI configuration webpage that is built in.
In a mountaintop to mountaintop setting, line of sight, these units easily will work over 26 miles (42km) (at full power) in the AirMax (TDMA) enabled mode.
These units can be configured as transparent layer 2 bridges so these devices will appear as an extended Ethernet line to the computers at each end. This becomes important for many Windows and device operations.
These units are powered by POE (Power Over Ethernet) so only one cable needs to be dealt with.
Cost? About $80.00 (U.S.D) each. You'll need 2 of them.
Before you go out and buy these units, they require a steep learning curve to set them up. Good directions are available but settings must be thought-out and chosen carefully.
If you would like to read the setup instructions before you buy, clickHERE.