Remove untrusted certificates

The untrusted certificates are on the website or other content that you are trying to view, not on your phone. You can’t remove them, you can just not visit that site, or click Trust if you are sure the site is legitimate.

Sometimes users get tricked into installing a rogue calendar, and they have certificates that can expire. If this is what happened, in the future beware of "free" offers

See this Apple support link→Delete spam calendars and events on iPhone - Apple Support

or watch this companion video:

Delete Calendar Spam

It’s probably a subscribed calendar, mentioned above.

Consider updating that iPhone 7 too, if it’s really still running iOS 13. Older iOS versions have older certificate definitions, older security, and known issues.

… Update your iPhone or iPad - Apple Support

… If your iPhone or iPad won't update - Apple Support