Elizabeth Cairnx

I can't see any answers here to the problem that a number of people have raised. I have tried phoning and chatbotting without success. I have LPA for my brother's finance and property affairs and need to complete his self-assessment as he is now in a nursing home. I have the access code for HMRC to see that this is all kosher but I can't find anyway of being able to log in and complete his self-assessment unless I impersonate him and borrow his phone to confirm the identification process. Obviously, I don't want to do that so I hope HMRC can give me a legal means to log in on his behalf.