Unfortunately all disconnections and game crashes counts as leaving the match and will be penalized appropriately. The game server has no way to discern a legitimate disconnection or game crash from one that is forcefully caused by a dishonest player to bypass leaver penalties. You can learn about this policy here:
Since you have already pinpointed the issue to your internet completely disconnecting I will spare the troubleshooting speech. Nevertheless, whether it is your intention or not, you did leave the game.