So you have an unlocked phone and have purchased a SIM card in Europe. Now you just need to install it in your phone. While the installation process is different for every device, the process should be as simple as powering off your phone, locating the SIM compartment, popping it open, ...
The same was generally true for MEO, even in the countryside and small towns when I was walking the Portuguese Camino de Santiago route. I won’t say there was nowhere without service, but given I usually kept my phone in flight mode to conserve battery anyway, I didn’t notice any blac...