My application was only in portrait mode, but I need a controller to also support landscape.

Here my info.plist:

    <key>UISupportedInterfaceOrientations</key>
    <array>
        <string>UIInterfaceOrientationPortrait</string>
        <string>UIInterfaceOrientationPortraitUpsideDown</string>
        <string>UIInterfaceOrientationLandscapeLeft</string>
        <string>UIInterfaceOrientationLandscapeRight</string>
    </array>

And in my app delegate I have added this:

var shouldRotate = false

    func application(_ application: UIApplication, supportedInterfaceOrientationsFor window: UIWindow?) -> UIInterfaceOrientationMask {
        return shouldRotate ? .allButUpsideDown : .portrait
    }

In my controller:

    override func viewDidLoad() {
        super.viewDidLoad()
        let appDelegate = UIApplication.shared.delegate as! AppDelegate
        appDelegate.shouldRotate = true // or false to disable rotation
    }

But the controller not rotate, what is my mistake? This controller is open in modal on a navigation controller

Past in controlllers seprately, According to your requirment.

override public var shouldAutorotate: Bool {
    return true
} 

override public var supportedInterfaceOrientations: UIInterfaceOrientationMask {
    return .all
}

override public var preferredInterfaceOrientationForPresentation: UIInterfaceOrientation {
    return .all
}

this should be in viewcontroller, where you want both landscape and portrait.

// other Viewcontrollers :

override public var shouldAutorotate: Bool { return false }

override public var supportedInterfaceOrientations: UIInterfaceOrientationMask {
    return .portrait
}

override public var preferredInterfaceOrientationForPresentation: UIInterfaceOrientation {
    return .portrait
}

// add this in viewcontroller ehere you want Application in portrait oreintation.

Enable all required orientation for whole Application.

And please remove your code in appDelegate related to orientation