Manually adding bank transactions to a linked bank account

I linked my bank account, however, my bank only provides activity up to 3 months, so the activity prior to this (about 1 month of activity) isn’t imported. The weird thing is the current bank balance is correct. How does it have the correct balance, but doesn’t have the activity for the first month? I attempted to add these transactions manually, but it won’t let me select an Account to link it to my bank account. How do I get these transactions in there? Also, how do I do a reconciliation? If I click on the bank balance in the Balance Sheet, which is the correct amount, it takes me to the transactions. The total of all the transactions equals the amount of the 3 months imported, not the balance, so there is a difference. Thanks!