Just to clarify, are you saying you need to cancel a pending plan change on your account?
If so, we'll need to access your account for that. You can contact us here to assist you or we can send you a PM and help you through the community.
As for the app login, can you confirm the following please:
Are you getting an error message when you try to login?
Also, do you login with your email address?
Are you able to login to My Account through our website?
Let us know, we'll be happy to help you further!