Using a computer, try to login to your Tesla account. Assuming that you're entering your credentials correctly using your iPhone, if you can't get in through the phone app, I'm betting that you won't be able to get in using a computer. At least, that's the case most of the time. There are times that I'm not able to login using my phone but can login with the computer, but such outages are rare and don't last long when they do occur.
If you are unable to login to your Tesla account by using a computer, you should go through the password recovery process. Once you are finally able to login to your Tesla account using your computer, try to do the same from the phone app. I'm betting you'll be able to get in.
Let us know how it goes... if you're still not able to get in, there are a few other things to try... this just happens to be first in line.