Haha don’t worry, it can be confusing! There are a lot of different mobile apps too (see lemmyapps.com and filter by your phone type) but it sounds like you’re trying to add a second account. You can only log in to your account on the instance you signed up to it with, then you search for the communities you want to follow.

Some apps let you log in to multiple accounts at once, in which case you could sign up for a new account at fedinsfw then log in to that account in your app, then you’d have two accounts you can switch between.