Wifi traditionally used the 2.4GHz frequency band, which is fairly good at getting through walls. However only a handful of devices can use 2.4GHz before it gets ‘full’ and becomes slow, so Wifi has been moving to 5GHz which allows for far more devices to communicate and at much higher speeds, but 5GHz doesn’t go nearly as far and really suffers with walls.
If you have access to the router you could have a look at the settings and try disabling 5GHz. Now the flip side is you will no longer get 400Mb/s in that room, it might drop to 50Mb/s, but you should see that 50 available more broadly throughout the house. If there are no other houses nearby using 2.4GHz this will work well. The other option, which is the better solution but comes with some cost, would be to add some more wireless access points around the house using 5GHz - this will give you the higher speed advantages of 5GHz more widely spread. This could be achieved with some network cables run to other rooms and access points plugged in, or using Ethernet-over-Power devices with Wi-Fi built in, or ‘mesh’ access points which connect to each other over wireless to get signal back to the main router.