If you still insist on getting Splatoon despite having bad a Internet Connection at your home, then you could try to buy some Splatoon amiibo, provided you can find them. Those will extend your single-player playtime by an hour or so each.
Otherwise, I would strongly recommended getting Super Smash Bros. 4. Loads and loads of replay value on solo and with friends locally.
I wonder on how bad your Internet actually is. You should take a test from
SpeedTest and choose the closest server based on where you currently live. For example, mine is...
I have a very bad connection and I still can play online Nintendo games decently enough, because it's surprisingly consistent for the most part. The only problem I would have is whenever I face with anyone outside of America or someone else with a much more unstable Internet. When I try to connect with Europeans and Japaneses servers, I always have a ping of 100+m/s, which would put me at a disadvantage. For everything else than online gaming, it's a perfectly serviceable Internet, but I don't expect being able to stream myself playing a video game because of the atrocious upload speed.