CC2340R5: Read the RSSI at a proprietary link

Tool/software:

I use the simplelink_lowpower_f3_sdk_8_10_01_02 and build a proprietary point to point communication. 
I want to read the RSSI of the rf link therefore I tryed to read out the value inside the callback function.

But I always get the value 127 which means invalid.

What am I doing wrong?

void echoCallbackRX(RCL_Command *cmd, LRF_Events lrfEvents, RCL_Events rclEvents)
{
     int8_t TempRssi;

    if ( lrfEvents.rxOk )
    {
         TempRssi = LRF_readRssi();

         if(TempRssi < 127)
         {
            RxFctRadioMessage.RSSI = TempRssi;
         }
    }
}