How do I check my past usernames if my account is locked?

My account has been locked due to suspicious activity but the only reason is because I was logged in into multiple devices on that account. But the Clash Royale support has been terrible and not helpful at all. I have all the details to my account, payments, and I even have all the names of devices I was logged intro. But all of that doesn’t matter if I don’t have my old username and I don’t know how I can possibly find it because I changed it so long ago. I need help so any help I get I greatly appreciate it.