How do I remove untrusted certificate?

Is this your corporate iPhone? Do you have any profile installed by your corporate / organisation / enterprise / School Management then please contact the system administrator of your corporate / organisation / enterprise / School Management

If you have installed any profile on your own --> Install or remove configuration profiles on iPhone - Apple ...

If you want to turn on SSL trust for that certificate, go to Settings > General > About > Certificate Trust Settings. Under "Enable full trust for root certificates," turn on trust for the certificate. Apple recommends deploying certificates via Apple Configurator or Mobile Device Management (MDM).

Trust manually installed certificate profiles in iOS and iPadOS ...