GL868 Modem Reboot problem
Found it! I was not giving enough time for modem to initialize after reboot so that it can find the right operator to register to. It takes some time for modem to register to the network. So when we issue AT+CREG? command to modem it will reply back with +CREG 0,2 indicating it is searching for operator for registration. After reboot of modem, we need to wait for some time before we can start using modem or we can continuously query for registration using AT+CREG? until we receive +CREG 0,1.