iPhone XR still shows it is managed by my former employer

I was allowed to keep my company phone (iPhone XR) as part

of a retirement package. I setup a personal account with AT&T as instructed

by the company. It still shows it is managed by my former employer, and it does

not allow me to connect it to my PC. I contacted the IT Department and was told the company has released the phone with Apple. How can this be corrected? A factory

reset?

[Re-Titled by Moderator]