POA form 2848

Hey, if my father in law has dementia and my husband is POA how does my husband sign the 1040? Does he sign as Trustee or POA after his name? I see form 2848 but it says the taxpayer has to sign page 2 line 7. If the taxpayer has to sign the 2848 what's the point of form 2848? He can't sign anything. Or do I need another form? Or do I just have to attach the full POA to the return? And this is a Joint return with mother deceased in 2020.

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