I'd like to explain this a little bit as I have the same internet based on 4G data
It's almost instinctual to think a PvP game would eat up tons of data but that simply isn't the case. This is why the initial download for league is so big, so you don't have to "download" all of the effects while ingame... That's very inefficient!
Most of the things such as the map, champions, etc are all included in the download and updates.
Most of the "internet" things that happens in a match is pinging. Everytime there is a movement command sent by a player a very small amount of data (a few kb at max) is sent to the server. For a 30 minute game this can add to a substantial amount of megabites, but not a LOT.
The most memory intensive things happen while loading extra effects like special ability effects, but this is awesome because it was already downloaded off your client when you installed league!
Just random fun facts hah