are you able to connect to a web werver successfully from another device in the same network?
Am i being MitM'd?
Yes it sounds suspicious. It would be beneficial for you to try running your browser through a remote server using SOCKS over SSH, so you can guarantee your traffic is encrypted and not being tampered with.
What I would do establish an SSH tunnel to a remote server with dynamic port forwarding enabled, then point your browsers proxy settings to the port.
This way if everything starts working like normal, you'll be able to determine if you're being MITM'd, you'll have a clean/safe connection, and you'll be able to take further steps on discovering where the threat lies.
Use traceroute, and set up netcat listeners on port 80 at different points of the network and connect to them and see what happens. Basically calm down and diagnose the issue.
If you don't have the technical knowledge to do it, contact your ISP and use a VPN in the future or switch providers.
401 is just the Unauthorized status code for HTTP. It means you do not have authorization to access the site. A quick web search would be sufficient to figure this out.
>Buy a wireless router and disable wifi
Literally what