iPhone 6 stuck in guided access, can't turn off, don't know passcode

My daughter accidentally enabled guided access on her iPhone 6. She also seems to have set a six digit passcode for guided access which she doesn't know, and we don't either. The phone can be restarted but not just shut down, it restarts automatically. When it restarts, it goes straight back into guided access, stuck on the iMessage app.

What I've tried:

I have tried plugging it in to iTunes and have backed it up, thinking I could restore it but it asks me to turn off Find My to restore the phone. I can't turn off Find My because the phone is stuck in guided access. There doesn't seem to be any way to turn it off other than on the phone itself.

I've tried to mark it as lost in iCloud, but some kind of error occurs and the message pops up that it 'a problem' occurred and it won't allow me to mark it as lost, either through icloud on the computer or through the family settings on my phone apple ID. The phone does not have a passcode set, only guided access has one (which we don't know).

I've tried repeatedly guessing the passcode with no luck. :-(

Any help is very welcome! I have literally tried every solution I can find online and nothing works!