Why my ZTE Grand Memo don't read the SIM CARD?

How do I take the back off my phone to take the battery out

Mine is a ZTE older model won't read Sim card. AT&T was to send me a new one but haven't received it can't get customer support to help me they won't unlock the phone so I can use a different carrier. Now someone got into my phone encrypted it. Also I have no data settings. 3 months in a row AT&T has done nothing and the problems ate getting worse . I have never seen such poor trained support. I can't use the phone no one can hear me . Any suggestions?