Module Xbee PRO S2C

Hello community
I’m working with 5 xbee pro s2c, which according to the specifications has a range of 60m indor and 3200m outdor.
I have configured one as a coordinator and the other as routers, in a topology similar to the star where my coordinator is in the center and routers forms a frame.

Are installed in a sorghum field, where it is assumed that its range is 3200m because it is in the open field or field, the module more far from the coordinator is about 320m and can not respond to my call.

The coordinator calls the routers by a code according to which he wants to talk to him and is secondary, first send the character to the number one who is @, once the coordinator reaches the router called “@” calls the second call “$” Then the third “%” and finally “&” all this in a sequence per minute, I mean with this, let’s imagine:
Minute 1 “@”
Receives string and shuts it with a “/”
Minute 2 "
Receives string and shuts it with a “/”
Minute 3 “%”
Receives string and shuts it with a “/”
Minute 4 '& "
Receives string and shuts it with a “/”

The firmware is ZIGBEE TH PRO 405F

How high off the ground are the antennas?

According to this Fresnel Zone calculator (https://www.everythingrf.com/rf-calculators/fresnel-zone-calculator), the antennas would need to be almost 8 meters above any obstacles to have a completely clear RF signal.

Other interference, such as weather or ambient RF signals, will also reduce range.