Do I need guardianship if I have power of attorney?

Due to progressing dementia and a stroke this past year, my husband is no longer able to make decisions about our finances, home, etc. We have a durable power of attorney document in place, but I'm not sure how to "act" on it. Do I need to do anything to "take" power of attorney? Is taking guardianship necessary if I am his POA? Currently, if he says yes to things on the phone, or signs something someone brings to the door, I don't think I have any legal way to show we should not be bound by his decisions. I appreciate any advice you have.