Security compromised on iPhone 6s

Please can anyone help, I’ve ‘lost’ my Apple ID to someone in China it appears. I live in the UK. Three days ago I started getting pop ups asking me to verify my ID with a Chinese telephone number. I also got an email purportedly from Apple telling me my account had been accessed by a user with an iPhone 5. I tried for two days to speak to Apple support and eventually was successful, after two hours on the phone they gave up on me, said to get a new email address. I still can’t access iCloud because it’s linked to someone in China using my ID. I’m at a loss as to what to do. Should I reset to factory settings? If I do will I lose my photos, etc? Thanks for any advice..