Registration via AT — attach, PSM, eDRX configuration

The Attach Sequence

Bringing a cellular IoT device from power-on to fully-attached takes a specific AT command sequence. Step 1: AT+CFUN=1 — enable full functionality, powering up the radio. Step 2: query the modem identity and capabilities — AT+CGMI, AT+CGMM, AT+CGSN for serial number. Step 3: AT+CGDCONT=1,"IP","<APN>" — define the PDP context with operator-provided APN. Step 4: AT+COPS=? or AT+COPS=0 — allow automatic operator selection (preferred) or list operators. Step 5: wait for AT+CEREG (or AT+CGREG for GPRS, AT+CREG for legacy GSM) event indicating registration. The unsolicited event "+CEREG: 1"…