At last Cell C are starting to fix some the cons they've been pulling on their users.
The 16 down, 3.6 up you've seen up to now was a artificial limit Cell C imposed on your connection. Even if you were under optimal conditions, you would never get better speeds than this.
This was a bit of a problem for Cell C as they've been telling everyone, at every conference and everywhere in the media you can get up to 21.6Mb/s. They even created a TV add showing speedtest.net running at 21.6Mb/s.
Meanwhile, without telling anyone they imposed this throttling on the network. So, even while flighting ads showing downloads of 21.6Mb/s, they knew it was impossible. In other words, they intentionally lied to you, their loyal subscribers.
I assume after the ASA wrapped them over the knuckles for lying about a bunch of stuff, including speed claims, they fixed this quickly as the ASA are now considering sanctions against Cell C for their continuous lying in the media.