There could be a fault with your NBN link and the modem is on 4G backup. On 4G backup speed is limited to 6 Mbps down and 1 Mbps up. If you have a Gen 2 modem the front light will be blue on 4G backup and if you have the Gen 1 modem the on line light will be purple when modem is on 4G backup. Even if not on 4G backup the speed should be greater than 4 Mbps. Suggest you ring Telstra NBN assurance 1800 834 273 and lodge a fault.