Answer: please check the following AT commands:
AT+CSQ? ----To check if the signal strength is too weak.
AT*PSRDBS=29 - —To select all the bands: GSM 850, GSM 900, DCS 1800 and PCS 1900.
AT+COPS=1,2,"" ----To select the operator manually. For example, for Claro Brazil, if MCC is 724 and MNC is 05, then you can input <AT+COPS=1,2,“72405”>.
For the MCC and MNC, please refer en.wikipedia.org/wiki/Mobile_Network_Code, or contact your operator.