Welcome to TPG Community!
Turning Off the modem if you are not using it, is okay. It will not affect the service or the device.
Pressing the tiny reset button on the modem will set all of the changes that you've made (wifi name and password, etc.) back to its default settings.
You can turn it off by using the power button, before disconnecting it from the power outlet.
Should you require further assistance, please let us know.