As the title say I can successfully put the GSM module of the Adafruit Fona Feather to sleep with either AT+CSCLK=1 or AT+CSCLK=2.
After that, the only way to wake it up is by a reset. It does not respond to:
1. DSR put low for like 50-100 ms for sleep command AT+CSCLK=1
2. Any AT command for sleep command AT+CSCLK=2
For 1 above:
#define FONA_DTR 5