The person who installed it said I need to be connected with ethernet, but I was wondering if this was a limitation on the router?
Sounds like you are connecting via wifi instead of with a cat 5e (or better) cable attached to the router. With the latter, you will probably see better speeds, although don't expect to ever see full 1 Gbps, probably somewhere in the 900s at best. Over wifi, even under the most ideal circumstances, you are unlikely to even approach gigabit speeds. For example, many laptops have 2 wifi antennas. With 802.11ac 5GHz using a 40Mhz wide channel, the absolute best wifi speed you could expect under absolute optimal conditions is ~360Mbps. But likely much less because of interference, signal attenuation, etc.
Disclaimer: I don't use the sonic supplied router/access point, so I don't know if it has inherent limitations. But I suspect if it does, that isn't the cause of the speeds you are seeing.