Can bank refuse DPOA for trust account if I'm not a trustee?

Mom has a trust. Her bank accounts are in the trust with her as trustee. I have DPOA. Mom wants me to use it to write and sign checks for her and otherwise manage the accounts. Legal department of bank said since I am not a trustee of the trust, they can't honor the DPOA. Lawyer said they can add me as trustee. Mom agrees, but not sure I want to do that. I am executor of estate on the trust. She can still sign checks, but what if she loses ability to do so? Does bank have to accept DPOA for healthcare?