When you visit your parents Is it just a day trip for a few hours or if it for a few days or longer. If it is a few hours then suck it up with no or slow Wi-Fi. If it is a few days and you must have it then offer to pay your parents the additional $30/mo to upgrade. Sorry to be cynical but they are saving money and it seems the issue is more on your end than theirs.
+1
If what they currently have works fine for them, I think it's unfair to ask them to provide anything more. If you really need faster internet for a portion of your visits, can you go to a nearby Starbucks or Panera for the morning or afternoon?
ETA: when I visit my in-laws, not only do they not have wifi (the horrors!!!), but there's also no cellular coverage at their house. I mean, I can get one bar if I stand on my head while holding my phone between the toes on my left foot if they're wrapped in foil, but for all intents and purposes, there's really no coverage. We just detach from the busy world and enjoy the visit.