In most parts of the world, 2.4GHz industrial wireless radios are available as unlicensed radios.
With a high gain directional Yagi antennas on both the ends that are pointed at each other, 2.4Ghz is likely to work at 2Km distance, if each antenna is visible to the other (open line-of-sight).
For wired communications, I rarely recommend Modbus ASCII, but wireless is one case where Modbus ASCII can make sense, if the radio will support it, because Modbus ASCII has far less stringent timing requirements than Modbus RTU. A Modbus ASCII message takes twice as long to transmit as a Modbus RTU message, but the less-stringent timing restrictions mean that more Modbus ASCII messages can be completed than RTU in difficult conditions when radio reception is marginal.
The catch/caveat is whether the radio supports Modbus ASCII. Modbus ASCII is, by definition, a 7 bit data word. Many wireless devices will not support 7 bit data words. They all support one start bit, 8 data bits, but many do not support 7 bit data words.
If the reply messages are short, like only a couple register values, then RTU might work, but make sure you can set the time out on the master to be fairly long (one or two seconds?), just in case the whole query/transmit/receive/reply sequence takes more than a typical wired 20mS.
Be sure that whatever radio you consider that it is a radio designed specifically for Modbus. Modbus radios send entire messages without interruption, but simple serial radios will break a message up into separate pieces and Modbus does not do well be re-assembled from pieces, particularly Modbus RTU.