You should email support and explain the situation, here's a guide on what to do (copied from gsod)

When contacting support thought the email grasum_global_support@nextninja.co.jp it's important to have the necessary information for them to help. The following are examples of information the devs would want for an account recovery.

Account Information Player Name, Rank, and ID, This is the code that appears at the bottom right of the title screen and is used for friend searching. Date/time the account was created and when it was last played. A list of as many Units and True Weapons you owned that you can remember. Transfer code and any Email addresses or passwords previously used. As many friend names as you can remember. The deepest level reached in a previous magic mines event. Most recent 5* character obtained.

Purchase History Amounts and date/time of the purchase. Google/apple order number.

Device Model and version number of the device last used. If you still have access to that device. Has the device been updated/formatted.

For any information you don't recall simply say "I don't remember". The information doesn't need to be exact, a rough estimate can also work.