I blindly recommend Carlos, he was a wonderful host and his apartment is great ! Wonderful view, big living room, many equipments and good internet connection. We were three and have stayed there for one month ! Carlos is very responsive and by far the best host you can have in Lisboa.
If I had to share some remarks: it is in front Lisboa main station so it is very convenient for transport but not the cutest area to live in. Another minus point would be that the apartment is not well soundproofed so we tend to hear each other a lot particularly in the morning when one of us would have liked to sleep
Otherwise, go for it ! it is a great accomodation !