You have asked the same question in an issue on the mcci-catena/arduino-lmic GitHub, yet here you refer to the matthijskooijman/arduino-lmic GitHub. So which of these libraries are you using?
Also note that the library likely returns an invalid RSSI value. A bug has been raised in mcci-catena/arduino-lmic GitHub. Most likely the same bug exists in matthijskooijman/arduino-lmic as well as that code hasn’t been touched anymore for two years.