If your iPhone is managed/administered by an Organisation / Corporate/enterprise and if they have installed Device Management Profile then these links will help you.
If your iPhone is individually owned and you have installed an app which prompted you to install a profile then these may help you.
Otherwise, these will be for academic purposes only
Get started with a supervised iPhone, iPad, or iPod touch - Apple ...
Install a configuration profile on your iPhone or iPad - Apple Support
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 ...