BMO Harris Bank Account Or Service Complaint

BMO Harris Bank account or service department,

Savings account Account opening, closing, or management District of Columbia

I had an account for around $1000.00 at - at the -, Wisconsin branch before that bank was purchased by BMO Harris bank. I was unaware that the bank was being purchased, and I was living abroad for several years before learning about the purchase. I then contacted BMO Harris bank to retrieve my money, and they have told me that they have no records of this account. They instructed me to contact the office of unclaimed property in the state of Wisconsin, which I did. They had no records of this account either. I have a banking statement of the account in question from -/-/-, but BMO Harris refuses to honor this issue. Please advise.