I received my cosmo on monday \o/.
This is indeed by design. You have to hit force rotate in the App Bar to rotate to portrait. The way to go back to Landscape is to hit the App Bar again. If you want to handle the rotation yourself, you can install a 3rd-party app for this. I don't remember which one works best, but there is discussion about this in the Gemini forum.
While not autorotate, a shortcut Fn + Shift + R rotates the screen. By design, hitting the Planet key always reverts the screen to landscape. Shown on one of the latest PC videos: https://www.youtube.com/watch?v=CbQQP59ULPA&t=98s
This makes one-handed (obviously right-hand only) use quite a bit harder than it already is (since the cosmo is hard to open with one hand). Ok, I have large hands, so my personal workaround is to
1.) Switch to Portrait mode before (!) anticipating one-hand use.
2.) Open with one hand (aided by other available body parts and being careful not to drop the Cosmo)
3.) Configure a fingerprint on the *right* hand for unlocking the cosmo.
Switching (!) to landscape mode with one hand is extremely hard (reach around to the planet key with right middle finger, then slide cosmo back and tap on "force rotate" with right thumb) and carries a high risk of dropping the device, because there isn't even a rotate setting in the pulldown settings or in the codi settings.
The OP said that "Ultimate Rotation Control" did not work and my impression from reading the gemini forums was that this was the tool of choice, so I don't know what might help here.
Left-handed use would require 180° rotation, which is simply impossible.
I have to admit that one-handed use even with my large hands is quite hard, because the cosmo has a large display and is quite heavy, but I find it hard to forgo in some situations.