Slow Ethernet, fast WiFi

Another thing maybe to check….it could be the router and gateway are broadcasting on the same channel numbers. Your router should have the ability to manually select channels for the 2.4 and 5 Ghz bands, so make sure they are different than the channels the Gateway is using.  You can see what channels are being used by downloading a Wifi analyzer for your phone.  If your Router is on a WiFi 6 mesh, you also have to worry about that channel the backhaul channel is using.

Lastly, if your devices are latching onto a 2.4GHz band on your router, you’re not going to get speeds much over 100 Mb/sec anyway.