Register a POA with the HMRC

[cityboy54]

I've now had 3 attempts on 3 different lines to register a POA for my mother (she has dementia) so I can manage her tax affairs.
Their website states the below (my POA was only registered 4 months ago) - but the agents keep telling me I need to write in and send a certified copy of the POA ...which seems to total contradict the guidance on their website re online access codes? (which I have)

Tell HMRC about the power of attorney by phone
The person helping you can tell HMRC by phone if all of the following apply:

the LPA was registered on or after 1 January 2016 in England or Wales

they have an LPA online account

they can make decisions about property and financial affairs

The person helping you will need to contact the relevant HMRC helpline. For example, they need to call the National Insurance helpline if they’re calling about National Insurance contributions.

When they call, the person helping you will need to give HMRC an access code so they can check that the LPA is valid.