Speed test measured from the router is the aggregate speed from the router to the Internet, while speed test measured from your mobile or laptop is limited by the device Wi-Fi performance, which is usually lower than the one supported by the router. For example, most current laptops cannot exceed a speed test of 500 Mbps even though the line rate from the router is more than 1Gbps. Furthermore distance of devices from the router or MESH also affect performance.
To get an accurate understanding of your broadband speed please use the GigabitIQ app, and follow the youtube video: