Remove iPhone being supervised and managed by previous company

Upon leaving my former company, I asked to have my personal # ported, but instead, they gave me the phone and ported the device instead. In settings it says, "This iPhone is supervised and managed by {company name}." I need to remove this because it's causing issues with multifactor authentication with my new company. I called my previous company, and they said the only way to remove it is to wipe it. I've completely wiped it twice, but this still persists. Apparently, this setting must be part of the back-up. How do I remove this?