Upgraded to new modem, but same slow wifi issues remain

300 to 400 Mbps sounds right for Wi-Fi. It will drop as you move away from the router. The speed will halve on each hop of a mesh network, as each packet has to be re-transmitted along the route.

Remember, Gigabit service is meant to offer reasonable speeds to multiple devices / users on your network, rather than super high speeds to one user. Many content suppliers cannot provide their data to consumers at GB speeds.