I'll give you several solutions, and soon we'll know why your cell phone automatically disconnects, without you doing anything. Here are a few options:
1-.Try to reset the phone to its default settings
2-.Start the battery, wait a few seconds, insert it and turn the device back on
3-.Try to perform a software update
4-.Leave the device to the store, for technical service to check it.
5-.You can also try to root, to see if this works without giving you that problem.
I hope I have helped you, my friend!