You should definitely be getting speeds closer to what's advertised in your package and we'd love to work towards a resolution!
In terms of modem swaps, it may indeed be the modem at fault. The swaps are done via tech support and the new modem would be sent by mail. Would you be able to speak with tech support to see if they can swap it out? You can reach them directly at 1-888-259-3436!
I hope that helps out, they'd really be the ones best equipped to figure this out for you.