Can't cancel the contract due to false advertising

I took Home office 20GB contract 3 months ago with LTE router hoping to improve my very slow standard Telkom internet line. I was approached through their call centre and after checking all necessary details I agreed to take the contract. The agent checked my previous average usage, which aparrently was between 12 and 13 GB/month. Having that in mind 20Gb should be more than sufficient. Also, I was insured the inside range should be 30m to use across other devices inside the house. Both of those were not true. Since I started with it, I've been running out of data half way through the month, leaving me without internet for more than 10 days every month. Last month I had to purchase extra data which added quite a chunk in the final amount I had to pay Telkom.<br> On a top of that, I can't even see it on my network on the other side of the house, which is more or less 15m away from the router.<br> When I approached Telkom wanting to cancel the contract, even if I had to pay some penalties, I was told that I would need to pay full outstanding 20 month amount to get out of it, which makes absolutely no sense.