Declaring / starting an activity (as self-employed, non-EU residents)

Hey folks, would appreciate your advice on declaring / starting an activity as independent/self-employed.

We (myself and my partner, both Canadians) are in the process of acquiring residence permit and need to declare / start an activity. We already submitted our applications for temp residence, had the interview at SEF in May and need to provide proof of declaring an activity - the only remaining document we need to submit to complete the application process (and, hopefully, obtain our residence cards).

We do have our NIF numbers with a representative, as well as bank accounts/IBANs in Portugal (also verified in the Financas portal) and the passwords to the portal. However, as Canadians, i.e. non-EU residents, we are restricted to complete this online, and were told we still need a representative to declare "start of activity" at a local Financas office. Our present NIF rep is a retired person, and apparently cannot act as a rep for starting an activity.

Not sure why going though all the steps (obtaining residence visa - which included a detailed business plan, arriving in Portugal, submitting the application with all required docs to SEF - except for the 'activity') would still restrict us to simply declare start of an activity. A chicken and egg situation as it appears with the residence card this can be easily done online. Also, a bit of an awkward process as we don't have many acquaintences in Lisbon who know us well and can vouch for us.

If changing our fiscal rep for NIF and having the same entity be our rep for 'starting an activity' would make it easy, we will go for it,

Any advice to resolve this 'activity' dilemma, and referals, will be greatly appreciated!!