I'm setting up some security and fire monitoring at our Scout hall, using a phone hotspot to provide an internet connection (the phone also runs the Cleverloop Smokey App).  The power in the hall is only on for a few hours every day when the hall is in use, but is off the rest of the time.  I'm looking for an option to keep the phone powered for the rest of the time.  

I've considered using a power bank, but the ones I've had switch off when the phone is charged, and don't switch back on until the button is pressed.  Are there options that do something different?  

This is a temporary solution, as the power situation will change after a refurbishment.  I'm looking for something very low to no cost.