You can try to reset your iCloud password https://iforgot.apple.com/ If that doesn't work either then it's a paperweight and you can sell it for parts
reads as though it is possible using "checkra1n". But again... it might be easier to just buy something else :-/
(We had some company iPhones in our device lab, not on MDM, and people would attach them to their personal accounts, so this was an occasional hassle.)