LoRa devices have an RSSI register, there is a link to the devices datasheet on the RA01 link you gave.
The datasheet will tell you which register holds the RSSI value of the last packet and how to calculate it.
Have you checked the Examples folder of the LoRa library you are using for any examples of reading the RSSI ?
Indeed, in the code you have posted you are sending the RSSI to the serial monitor, so its not really clear what your question is ?
And an explanation of the project and why you want to read the RSSI values would be helpful to the forum.