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Very low RSSI values measured at RX - SX1276

I am seeing much less range than expected with a SOM built around the SX1276 part. I have a transmitter board wired up through an RF attenuator to a receiver board to validate the system and I am seeing RSSI values that are approx 30-40dB less than what I think I should be seeing.

When I wired up the transmitter to a spectrum analyzer through an attenuator, I measured power and was seeing the expected dBm.

This leads me to believe the receiver is not configured correctly. Are there any specific registers I should check? The module I am using says that the RF switch is controlled through the SX1276 part so I assume it uses the RXTX pin output.

Here is how we are configuring the transceiver:
spreading factor is 8, bandwidth is 500kHz, coding rate is 7, frequency is 920MHz. We’ve tried both enabling automatic gain control and explicitly setting different gain values, but the results were the same.

The RSSI values are read from the PacketRssiValue register in the transceiver, and computed with the equation provided in the SX1276 datasheet:
-157+PacketRSSI+PacketSNR*0.25(when using HF port and SNR<0)