GL7600 getting intermittent +CMS ERROR: 287 error

This problem still exists and is very inconsistent. Sometimes, my GL7600 can connect to 4G network, sometimes it can’t. However, if I use the SIM card on my iPhone 6s, I’m always able to connect to the 4G network. I need to repeatedly turn on and off power to the terminal until it can successfully connect - I know this when the LED light blinks.

I’m not sure what’s the problem. Is it a firmware or hardware bug or an antenna problem?

Will appreciate any help!