Connecting EC21, EC25, EG06 or Other Qualcomm based platforms.

Using EC21 example, the main AT commands for initial configuration are:

AT+CGDCONT=1,"IP","<APNstring>"  //reboot after any manual change of this
AT+QCFG="nwscanmode",0     // by default in auto mode, set to 3 to force LTE only
AT+QCFG="band",d3,8008000055,0,1   //restrict to  4G B1,3,5,7,28,40 - 3G B1,5,8 - 2G B1,8 for Australia, New Zealand networks
//These commands above only need to be set once and should be persistant across reboots.

AT+CEREG?       // Check for registration on LTE network +CEREG: x,1 (or x,5 for roaming) if success.
AT+CGREG?       // Check for registration on 3G/2G networks.

Note that Telstra APN may be telstra.internet, telstra.wap or in some cases just a blank string “” works too.
Optus APN may be yesinternet
Vodafone APN may be
If in doubt check with your SIM provider especially if using an MVNO that may have their own APN setting.
List of Australia MVNO.

If the module does not register on 4G(LTE) but registers on 3G/2G, this may be a sign that the APN is incorrect.

If your module is enabled for VoLTE (voice over LTE) then it may be that the APN is selected automatically when first reading the SIM network.

VoLTE (voice over LTE)
This function is the support for voice calls in LTE radio mode. If the SIM plan or module does not support this VoLTE service then voice calls started in 4G will automatically fall back to 3G in call. In this case if 3G service is not enabled in the module or not available, then the call will fail with error message.

AT+QMBNCFG="autosel",1    // Enables auto selection of internal operator-specific configuration by reading the SIM network.
AT+QMBNCFG="list"      // list of internal operator configurations
+QMBNCFG: "List",0,0,0,"ROW_Generic_3GPP",0x05010824,201806201
 +QMBNCFG: "List",1,0,0,"Reliance_India_VoLTE",0x05011B4B,201801051
 +QMBNCFG: "List",2,0,0,"TW_Mobile_China_VoLTE",0x05800301,201805021
 +QMBNCFG: "List",3,0,0,"Bouygues_France_VoLTE",0x05800101,201705111
 +QMBNCFG: "List",4,1,1,"Telstra-Commercial_VoLTE",0x05800716,202002061
 +QMBNCFG: "List",5,0,0,"Commercial-Smartfren",0x05012509,202001041
 +QMBNCFG: "List",6,0,0,"VF_Germany_VoLTE",0x05010444,202001021
 +QMBNCFG: "List",7,0,0,"TIM_Brasil_VoLTE",0x05800B02,201806061
 +QMBNCFG: "List",8,0,0,"VF_Australia_VoLTE",0x05800CE0,202002081
// Example above from E21-AU, shows the Telstra SIM has enabled Telstra specific settings to be enabled.