Please note that the mobile phone number serves as the 'username' when you log in. If you simply enter a new number, your account with the previous number won’t be recognized and a new registration with the new number will be started instead.
Therefore, first close the app completely, i.e. from the multitasking view, and restart the App to return to entering the number. Then log in with the old number.
You can then easily change your number using the PUK we gave you in the app settings. You’ll find the PUK in the subject of the 0.01 EUR transfer we sent to your withdrawal account. You received the first PUK when you registered, so this may have been a while ago. Note that each PUK expires after one use and a new PUK will be automatically transferred to your withdrawal account. So make sure you always use the current PUK.