gprs is just the name given to a object of the class CGPRS_SIM76_SSL which handles the SIM7600 module. I guess there is a bit of history there.:
CGPRS_SIM76_SSL gprs;
The actual getSignalQuality() method is here in SIM76_ssl_beelogger.cpp
/**************************************************************************/
/*!
@brief get signal quality on the given entwork
*/
/**************************************************************************/
int CGPRS_SIM76_SSL::getSignalQuality()
{
char buf[8] = "AT+CSQ";
sendCommand(buf,10000,","); // suche nach Komma zw. rssi und ber
char *p = strstr(buffer, "CSQ: ");
if (p) {
int n = atoi(p + 5);
if (n == 99 || n == -1) return -1;
return n * 2 - 114;
} else {
return -2;
}
}
It looks like it issues the AT command "AT+CSQ" and is prepared to wait up to 10 seconds to find a ',' returned. Once it gets that, it looks for what follows "CSQ". If that doesn't work it returns -2 (which you keep seeing).
What fixed power supply are you using? If that is not particularly strong, that would be one thing to suspect.
Also, what is connected to the Nano pin A2 ? This is in your test sketch #define GSM_Power_Pin A2
You can switch on enhanced debugging by changing the following '0' characters to '1'
in SIM76_ssl_beelogger.cpp
// Debug Information Datenempfang aktivieren, Serial.begin() muss in Hauptsketch
#define DEB_data 0
// erweiterte Debuginformationen
#define DEB_more 0
#define DEB_cmd 0
You may also be able to find some simple tests which involve connecting the serial monitor to the SIM7600 directly and issuing commands to test its network connectivity, say sending an SMS to your smart phone.