River sneaks out and goes on adventures all the time. Marco could do the same, and he' have the advantage of not needing to hide this from his wife because she'd be sneaking out for the adventures with him.
Also, given that Tom was able to buy those Mackie Hand tickets from his castle, most likely you can use magical devices to connect to the Internet. For that matter, since Tom will himself be King of the Underworld, Marco would be able to hang out with him all the time and they could pass it off as a diplomatic mission.
And given how often the lack of indoor plumbing on Mewni gets brought up, obviously that's one of the Earth technologies that Marco would arrange to import once he's king.