Typically this is a ScreenTIme issue but you say you have checked that.
If your Apple Account is grayed out in Settings, or you can't sign out - Apple Support
It could be SDP but I would think it would be clear if it was: About Stolen Device Protection for iPhone - Apple Support
Refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple Support - Apple Support For some countries look under “Other” listing at the end of each region. Not every country has telephone support.