I initially bought a NOS sim-card on ebay, their cheapest pre-paid tariff I believe is called smth like ‘Libre Dia’.
Activation is automatic, can top-up online etc. but I never used it outside of Europe.
This needs to be used every 90 days for a paid call/sms otherwise it will be blocked.
I also have a MEO pre-paid sim-card, this one bought in PT, their tariff works slightly better i.e. they automatically charge 1 euro for inactive account so less risk of deactivation for non-use.
A third possible option would be Vodafone PT, I have no experience with them.