Hey sauna_haltia,
This might not be the response you were hoping for, but I'm afraid the team needs to know the account's User ID in order to locate the account. Unfortunately, having the account's username alone isn't enough for our advisors to pull up the account, as several individuals could share the same name, making it an insufficient unique identifier. Having your purchase history and additional information is great for verifying ownership, but that's only applicable after the account itself is located.
That said, if our advisors have shared that they aren't able to locate the account, then sadly there isn't anything we can do for you here on the forums, as we don't have access to account information and can't escalate or investigate this for you.
We have more information on recovering an FC Mobile account here: How To Recover Your Lost FC Mobile Account. Thanks!