Speed Issues

I bought a 750S router for small apartment, which subscribes to 400 mbps download / 20 mbps upload. Upload speeds are fine through the router, but since the last firmware update, I am experiencing much slower download speeds when I Speedtest. In particular, my wired ethernet connection is even slower than my Wi-Fi connection on my Macbook Pro (through a usb c gigabit adapter). When I connect the gigabit adapter directly to my modem, I see the full speed of my ISP.

Were there new QoS settings that are throttling my speed? How can I get back the maximum speed I was seeing before the update? I have tried a factory reset but no luck. How do I start debugging this.