Twitter Revoke 3rd-Party Permissions from iPhone & iPad iOS9
You are on the right track with the desktop mode as it seems you cant do this from within the app.

As long as you can bypass that and go directly to www.twitter.com you should be fine.

To do that...
On Android, you hit the 3 dots near the address bar, scroll down, select "Request Desktop Site"

On an iPhone (currently at least!), Go to Twitter and scroll a little so you get the add new tab icon etc appear. Then, scroll across until you see the "Request Desktop Site" icon. Select it.

Now if you go to www.twitter.com and login you should be able to see the "apps" option in your settings and be able to revoke access.

I hope that helps!