Began with MEO -postpaid – covering a package of home internet+TV+mobile, and recently switched to Vodafone because of substantially better in-service provision. Cannot speak about NOS.
Vodafone’s pricing for mobile - 5G SIM + 1GB data is €20.50 per month to €31.50 per month for 10GB (called RED): Tarifário Red - Vodafone Portugal. Vodafone have an office at the airport and you can take out a plan for 30 days and see how it goes - or ask for more information. The airport staff speak excellent English.
With MEO, telephone only plan is €17.99 per month 1GB to €27,99/mês for 10GB. Tarifários de Internet móvel para telemóvel | Net móvel MEO.
With NOS, the cost varies from €16.99 (1GB) to €26.99 (10GB): https://www.nos.pt/particulares/telemovel/tarifarios/Paginas/pos-pagos.aspx
All the above prices require a 24-month contract, and you ONLY need a NIF and bank account, and will need to agree to direct debit. The contract becomes active once the bank verifies the IBAN and direct debit. Mobile providers are not terribly interested in whether you reside locally or not, but whether you are reliable in paying. In my case, an address was required for home internet/TV etc.
There is no unlimited data on mobile, so the cost depends on how much internet you think you are likely to use on the mobile- and obviously it’s good to have the same number for banking and other 2f authentication. If you are in Lisbon all the 3 providers work very well. I find MEO and Vodafone coverage excellent in Lisbon. Out of the city, the coverage varies a lot by provider. With MEO, I had free calls to US/Canada, and see that that option is not available with Vodafone.
Towards the end of the fixed contract, return to the provider and negotiate a better deal. Once MEO have you as a customer, they become complacent. I had no problems with their mobile tariffs/coverage, but they were a pain to call to ask to upgrade anything 