How do I disable the pop-up blocker in Safari on iPhone?

It may not be sufficient to "shut off" an Extension. Try removing it. No Safari Extensions should be installed unless you have a specific reason for installing it and you have familiarized yourself with its limitations.

Excerpted from Get extensions to customize Safari on iPhone - Apple Support:

Important: Check the extensions you have installed and make sure you’re familiar with what they do.

That Apple Support document also describes how to remove Safari Extensions.

Refer to Block pop-up ads and windows in Safari - Apple Support.

To allow pop-ups, Settings > Apps > Safari > General > Block Pop-ups > disable.

A Safari Extension may be interfering with that setting, so to rule out that possibility investigate Extensions under that same setting. By default no Safari Extensions should be installed.