I agree with this. As you're working in a small startup why not ask if you can just change your title? Unless there is some obvious hierarchy it couldn't hurt to ask to change it to say 'Digital Project Manager'. If you show willingness (and not ask for an immediate pay rise) there's no harm in them 'promoting' you. I know from working at quite a few agencies they generally love to show off their 'X Manager' to their clients. And the fact they got you on the cheap makes them warm and fuzzy inside. Once you have had a fair crack at the whip you can ask for the salary commensurate with your experience. And if they so no. You can go elsewhere and start your new journey.