Does your DD want to be able to sleep and is she able to describe what she does on her phone all night when she's awake? Given that holding her phone no longer seems to help her sleep, I wonder what she thinks might happen if she doesn't have it?
Assuming she does want to sleep, it might be useful for her to look at basic CBT ideas such as jotting down what she's thinking when she can't get to sleep, what emotions she experiences and the physical sensations she notices when this happens. She could then look at what she does as a result e.g. scroll through Instagram, look through X app for a suitable story, turn her pillow over, huff and puff etc... which then feeds back into the original 'I can't sleep' bit. There are various self help books out there that take readers through the process but that's only going to be useful if she wants things to be different.
Helping her to access alternative strategies during the night may well involve you trying out some of the ideas others have suggested e.g. finding out if her Alexa can be used to take her through a guided meditation, rehearse French verbs, read a story. If she will accept support from you to do this together during the day (and you provide a written list of tried and tested Alexa commands for her bedroom), perhaps she'd agree to try one of them if she's awake at 1am, if not sooner. Having a kindle with preloaded books or even borrowing a whole load of not-too-challenging books of her choice from the library and having them next to her bed might be another distraction to add to her toolkit... along with things such as fluffy socks to put on, a nightlight projector to create patterns on the ceiling, a white noise toy like those given to babies and maybe even a SIM-less Nokia 3310 that can only play snake but feels similar to a phone.
Her need for her phone overnight isn't going to go away quickly but as it's stopped working anyway, helping her to diversify her sleep props and supporting her to try them out over a period of time may be a useful way towards challenging her belief that only her phone will help her sleep. I also wonder if it might be useful for her to install an app that will keep track of what she's doing on her phone overnight. If she's being distracted by constant notifications, she could make a positive choice to set up a do not disturb period or even have the phone set to only allow sleep-enhancing apps overnight (asking you to set a password on the parental controls so she can't override it at 2am).
I do sympathise; 15 year old DDs are hard work!