Power of Attorney with Unknown account number - Need access to account

[mistervimes]

My mother has been moved to elder care and I need account access. 

• I have the phone, but it is a Kyocera flip phone with no data plan (cannot open links sent to the phone)
• I do not have the account number. I have no bills as is is auto-drafting from mom's bank account
• She has no idea what her PIN is. 
• I have power of attorney documents
• No one at the corporate store is able to help
• Calling the Loyalty line did not result in any help

Please assist.