How do I block a certificate that was somehow accepted?

That's usually associated with Wi-Fi captive portal.

If you're not on a network you need to connect to, you'll want to "forget" that Wi-Fi network, and not use it again.

If you're not trying to connect directly to a Wi-Fi network with a captive portal, are you using a VPN or similar, or are there any add-on "security" tools? (Your iPhone doesn't indicate a VPN is active, though.)

Any add-on network or content filtering tools present on the network you're connecting to such as parental controls, nanny filters, content filters, content loggers, or maybe is this a guest network?

If you are supposed to be connecting to that captive portal as part of your role within your organization and the organization's network, have a direct chat with the folks maintaining it.