This can be so many things and is the key reason it is recommended you never use wifi for online games.
I suspect your video streaming and downloads actually do see the lag spike. There wifi network does not know what is in the data packets. Downloading will just retransmit damaged or loss packets. Video streaming has a buffer to hide random variations in the delay.
Both packet loss and random packet delay cause large issues for games because they have no ability to try to hide the problem like other traffic can.
There was a issue with what is called wlan autoconfig that cause lag spikes on a very fixed time schedule. Used to be you could try to disable it but microsoft will just re-enable it now. Microsoft claims they have fixed the previous issues with autoconfig.
The more likely problem is it is interfernce from outside your house or even inside your house. Not much you can do about this.
For online games consider a ethernet cable if there is any possibility. Next you could look at MoCA if you have coax cables in both rooms . If not maybe look powerline networks since even though it is sometime slower than wifi it is still much faster than any game need and is almost immune to interference issue that wifi has.
Thanks, but a wired connection is not possible for me. Currently renting within a house.
I know wifi is not good for online gaming, however I did not have this problem for the past 4 years.
It only started happening maybe 3 weeks ago. It also seems to be getting worse each week.
Nothing has changed within this time period to my, or my landlord's knolwedge. I had also tried rebooting the router.