CC1310: More than 4 km Range could not be achieved

Part Number:CC1310

Hi

I am using CC1310-1190 Launch pad for my application where i am to transmit data for more than 10 km.

I am using following settings 

1.Data rate 5 kbps (Simple link long range mode)

2.Frequency 869.525MHz. Since launch pad is CC1310-1190-EU.

3.Rx filter bandwidth 49kHz

4.Deviation 5kHz

5.Tx Power 26 dBm

6.Whitening CC1101/CC2500 Comaptible

7.Antenna -PCB antenna as on Launch Pad 

I used excel sheet for range calculation provided by Ti which shows more than 10 km expected range for vertical polarization.

But in actual case where Tx is at 12m height and Rx at 1 m height LOS, range achieved is 3 km with CC1190 ON on both tx and Rx.

What could be probable reason for not achieving the range ? How i can i extend the range?