Accounts are bound to the servers they were created on (i.e. you created an account named "Hero" on server 1, "Hero" will only be accessible from server 1, selecting another server will create a new account).
If you are prompted to start a new account, restart the MHA: TSH game app and tap on the "Region" display to select your correct server. Recent Login will display your created accounts.
*If you are unable to locate your account, try logging into the server your account was created on.
Also, make sure you are logged into the correct account (Facebook, Google, Apple). Tap on the Account icon located in the top left of the Title Menu, and try all possible login methods.