I can't say for sure but looking at the specs, the 2920n has a maximum firewall throughput of 150mbps (~18-19mb/s) which would explain why you're not seeing anything like the full throughput. It's a 9 year old device trying to push speeds that it wasn't designed to handle.
If you want more speed, you need to upgrade - Draytek has a comparison table available (
https://www.draytek.co.uk/products/comparison). Ideally you're looking for one of their devices that pushes either 400mbps or more which will give you headroom for the future if the service receives speed updates