Having a fiscal representative resident in Portugal is obligatory when the person is the owner of a vehicle and/or a property on Portuguese territory, or when the person works on Portuguese territory (whether for a third party, or as self-employed). The obligation for local fiscal representation is optional if the person resides in the EU/EEA.
Exemption from the obligation to have a locally resident tax representative occurs when the person is (i) not a tax resident in Portugal or EU or EEA; (ii) does not fulfil any legal requirements of having a Portugal tax status; (iii) is not subject to taxes in Portugal; (iv) has no tax obligations nor intends to have any tax obligations in Portugal. You must be contactable, as persons with tax liabilities can disappear withouy trace.
Information from the source (Portuguese Fiscal Authority- Autoridade Tributária e Aduaneira) is at: https://www.apeca.pt/docs/informacaoapeca/oc_90054_2022.pdf . See this thread:
NIF: If no longer needed, must I still pay for fiscal representative and/or file taxes forever? - #16 by Onward. Your lawyer may be your tax representative, but you have the right to have direct access to the Portuguese Fiscal Authority portal.