you can get an antenna with higher gain. But it will be very directional (i.e. you have to point the transmit antenna right at the receiver for it to work at all).
Semtech makes a chip that works at this frequency range that would have the range you want, because they heavily code the digital transmission and make up the range in "coding gain". Look up any of the LORA modulated chips.
Also if it is one way communications, you could simply add a transmit amplifier. It is likely you are only transmitting at a miliwatt or so, so adding a 100 mw output amplifier would help a lot (assuming the country you are in would legally allow that effective radiated power).