How to remove unauthorized MDM device management and root profile on iPhone 15 Pro

There is no such thing as an unauthorized MDM. To install MDM, you either

1. Must purchase the device from Apple or an authorized reseller and have it added to Apple Business/School Manager and have an MDM connected for automated device enrollment
2. Someone must have physical access to your device and must have your passcode/password.
3. Someone must have physical access to your device, your iCloud password (assuming you've set that up), and be in a trusted location (or wait for an hour, again, assuming you set iCloud up), then they have to wipe your device and add it to Apple Business/School Manager manually.

Option 1 cannot be removed by you because you don't own the device.

Option 2 can be removed in Settings > General > VPN & Device Management.

Option 3 can also be removed in Settings > General > VPN & Device Management if it has been less than 30 days since the device was enrolled. You would likely notice if this one was the case as again, at a minimum, it requires the device to be wiped.

These are the only options.

The issue you're seeing when trying to sign in with a personal account to the work/school pane is a poorly worded error code from Apple. It does not mean your device is managed.

Just a couple of observations from the information your provided:

• Everyone has the option to sign into a School/Work Account at Settings > General > VPN & Device Management. When your device is not currently being managed, everyone will see that same message saying Sign in attempt has failed. You are supposed to see that.
• Managed Wifi networks are provided by your Cell Provider and certainly does not mean your device is being managed. Cell providers use these for Wifi when cellular is unavailable or to reduce cellular traffic. For example T-Mobile uses one to offer in-flight Wifi on participating airlines. Verizon uses them for their WiFi deals with many sports stadiums. Each cellular provider has their own networks they use and you will have a hard time finding a cellular provider that does not include them. Test it for yourself by removing your SIM and you will see those networks go away. You can contact your cell provider to see if you can opt-out, but that is not likely as you agreed to them with your cellular contract.
• When experiencing internet connection trouble, the first thing to do is turn off Low Power Mode. Since you are experiencing problems with the Spectrum Wifi, you should contact them. It appears it has not been completely set up properly with that network name and you likely need to do that with the Spectrum app. The signal strength with that network is poor that causes your connection issue.
• The one screenshot where it shows Sign Out is not available due to restrictions is most likely your Apple Account. You are either on a Family Plan or you have screen time restrictions on your device at Settings > Screen Time > Content & Privacy Restrictions > Account Changes set to Don't Allow. If you have trouble and it shows Allow then turn off Screen Time completely at Settings > Screen Time. If there currently is a passcode set for screen time, you will have to choose the options to change the passcode where you can turn it off. When you have turned off the Passcode, you can then turn off Screen Time at the bottom of the page.
• The message you see in your Certificate Trust page is because you are using the network provided by Spectrum as a HotSpot. For Wifi, you would create your own network with the router and connect to your Network, not one provided by Spectrum. The same thing is seen with Xfinity networks where you can connect their networks just like a HotSpot, but you actually should be connecting to your router for Wifi where it is your own private network you are connecting to. That is normal.

If you want to tackle one issue at a time, it is no problem, but nothing you have provided so far is any indication that your device is being managed at all by any MDM software.

I also reviewed the Spectrum setup for Wifi, and see where the install process is confusing as first they have you enter a SSID (Network name) and password for your own private network, then they are having you use the default network provided by your router with the SSID/Password on the router. This is the network name Spectrum Setup-AF, and should only be a temporary network when setting the device up to establish your own network, but you should be using your SSID/Password that you provided for your own private network.

https://www.spectrum.net/support/internet/self-installation-spectrum-internet-wifi-wifi-6-router

Bottom line is that if you are connecting to Spectrum Setup-AF, you are not doing it correctly and need to establish your own private network to connect to.