9xtend settings for distance?

I have a mobile traffic light system and am using 900Mhz modems over 1km distance in urban environments currently. But this modem is no longer available

I am evaluating 9xtend modems but I’m only getting 800 metres before the link becomes unreliable.
I am currently using 900MHz 6.5dB mobile phone antenna which worked with our previous modem the base of which is 2 metres high

I have RR=0 MT=0 CS= low FL=0 BR=9600 power is 1 watt.

Increase either RR or MT depending on if this is a point to point or point to multi-point connection. That should help. Also make sure that the antenna is properly tuned to the 902 - 928 MHz band. Most Cell antennas are not.