One leave will not cause a ban. You've been leaving quite a few games collectively and your previous game was the straw that broke the camels back.
Your internet died, it happens to the best of us, though your internet issues had a detrimental effect on the experience of 9 other players. While you were disconnected, your team was likely losing any lead they had accumulated and feeling much like you are now.
The only issue with the LeaverBuster is that it cannot differentiate between a client bug that occurred in the recent patch, an issue with the server and loss of connection on your end. Other than that, it's quite effective. If it's a large scale bug or a server issue, Riot support will no doubt lift the ban, so even that issue is only minor.