Using the docs here: https://developer.apple.com/library/content/documentation/AppleApplications/Reference/SafariHTMLRef/Articles/MetaTags.html
It seemed like you should ignore most of the viewport properties and just set user-scalable to “yes”. It’s working on my iphone now.
<meta name="viewport" content="user-scalable = yes">
Edit:: The mobile tester site doesn’t allow scaling, so it just gives scrollbars. With an actual phone this works.