The reason I said that I've had my S rank taken away from me so many times is because of how many times I have gotten a few points away from getting back to a S rank and have a teammate disconnect or a communication error during the match that would have leveled me up. That's what's getting me salty.
Still, it's just a few disconnections. I mean, obviously this is salty, but to go from A+ to S you just need, I don't know, maybe 55% victories. You have some disconnections ? Then get to 60%, and you'll just go S. Just improve a tiny little bit and you'll be a proud S, even if you have some disconnections - but don't let the salt go with yu in the following games. You lose one or two because disconnections, alright, but don't let it angr you or something, because this is what will make you lose games you should have won.
I know this may seem a bit "unkind", but think about it: what would you say to a B+ saying "I can't get A- because of some disconnections"? You would think "well, I have them too yet I still got to A+". And don't overestimate S or S+, they are only humans with lots of mistakes, just make them pay for them :)
About the internet, I have Ethernet and think it's better than when I was playing without it (I guess I get disconnected one out of 1000 games? While before I would say maybe 1-2 times a day, I don't really remember). Yet I can't garantee it will be good for you... Still it's worth a try, as you can find adapters not too pricy. Maybe you can try the "internet manual settings" things before, as it's free. Sorry I don't know how it's called in english, but it's about setting your connection all by yourself by following a Nintendo step by step tutorial.