threaten them that you're going to cancel your subscription until they offer some deals. It seems to work for a lot of people.
This. Speedtest was reporting 0.01Mbps while I was paying for 3Mbps. Called them, told them to cancel, negotiated for a new modem, 10Mbps for the price of 3Mbps of download.
I canceled after a while though. It was unreliable so I went with a more-expensive internet provider. Paying 12Mbps download, 5Mbps upload. Speeds are pretty static and the service was never down.
But try to negotiate with them.