How do you trust a 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 ...

My iPhone has no such setting. Under “Settings > General > About > Certificate Trust Settings”, it just has information about the Trust Store version and the Trust Asset version (screenshot attached). I’m running the most recent iOS (15.2) on an iPhone 12.