I have a Lightroom subscription for my desktop but it won't let me log into premium on my iPad. Do I need a separate subscription? My email address is different for my desktop and my Apple ID - I didn't really think about that when I subscribed. Should I change my email address?
You have to log in with the same account on both the computer and iPad. It has to be the same account, not a different one. The subscription you use on your computer is also good for your iPad. What do you mean that Lightroom on your iPad won't let you log in? How are you trying it? What happens?
No, do not change the email address on the desktop account. Instead use the desktop login to sign into Lightroom on you iPad. If you already had a Lightroom account for your desktop, you didn't need to purchase a premium account for the iPad.