Bank not honoring POA due to suspicious online banking transaction. Advice?

Sorry, but if you are invoking the POA then the client cannot make informed decisions so can't confirm u as POA.

I would first ask the bank what they need to honor the POA. May just be a letter from the lawyer who drew it up, if not available, any lawyer saying that the POA is now enforced. Lawyer may need a letter from doctor saying the person who assigned you is no longer capable of making informed decisions.

Thank the bank for looking out for the account!

Either take the client to the bank with your POA, if possible,
OR
Get the two of you on the phone with the bank, and start the process of identifying yourselves until the bank is satisfied.

More detail needed here.