I got it yesterday, installed GrapheneOS on it, went through all the settings, and it was fine. Now, when I try to turn it on with just the power button, nothing happens. I can hold down power + volume up and after holding it down for an awkwardly-long amount of time, it brings up the fastboot menu and says "this device has a custom operating system". I can press the power button to start up GrapheneOS perfectly fine, until the phone turns off less than 30 seconds later. Sometimes, when I'm turning the phone on with fastboot, the screen flashes completely green for a millisecond. There was one time when the fastboot said no operating system could be found.
I'd like to know if this can be fixed and how. If it can't be fixed, I want to know if I can return it or not. I live in the UK, bought it off Google's online store, and haven't bought preferred care.