I have a Mac running Mojave 10.14.5.

I have an issue with the home Wi-Fi. I have a 400Mbps internet connection. When I start the laptop, the Wi-Fi is fine and I'm getting expected internet speeds (as per fast.com), but at some point it drops to less than 100Mbps (again, as per fast.com), and somehow this affects all other devices (phone, tv) connected to the same Wi-Fi.

But then just after I restart the Mac, the speed goes to 400Mbps again, on both the Mac and the other devices.

How can I troubleshoot this situation?

Edit: I added and Image of how things look when connection drops.