As quoted from the linked post.
It looks like you’re part of one of our experiments. The logged-in mobile web experience is currently unavailable for a portion of users. To access the site you can log on via desktop, the mobile apps, or wait for the experiment to conclude.
This is separate from the API issue. This will actually BLOCK you from even viewing reddit on your phone without using the official app.
Archive.org link in case the post is removed.
You should be able to add it as an account, I think from the top left when you are the main feed.
Edit : went and checked, there’s a down arrow next to your name when you hit the menu icon, hit it and you should be able to add an account on a different instance