While it may not be your fault that your ISP decided to perform maintenance and shut down your internet, it did create a much more difficult game for your 4 teammates. This can be anywhere from ruining the only match a player has time for, to costing someone their promotional series, all of which was completely out of their control, through no fault of their own, and was something they bore no responsibility for.
Like the others have commented, I'm a bit confused what you mean by banned. It takes more than one instance of leaving a game to be issued a ban by the LeaverBuster system. A first leave/afk will result in a warning issued by the system, a second instance in a short time frame can result in a player being played into Low Priority Queue for 5 games with a 5 minute wait time. Note, some instances of leaving, if the player has done so in the past, or if it is a ranked game, will bypass the warning and progress directly to the 5 minute LPQ.
Continued leaves will increase the penalty to 10 minutes, and then 20 minutes, both at 5 games each. To even be considered for banning, you would have to have gone through at least 5 games of leaving/afking within a short time frame.
So, either you've had a lot of problems with your ISP, or you are referring to a low priority queue instead, or something else? If you are referring to the LPQ, Support is not going to take you out of it. While your ISP performing maintenance may have been out of your control, and may have been through no fault of your own, as it was your internet and your account that disconnected from the game, it was ultimately your responsibility. Any time we log into a match, we are committing to playing through to the end of the match, or are agreeing to face the consequences if we do not.
Edit: Fixed a number typo.