I would be glad to help. This error usually means that the server hosting the website is overloaded or unable to accept new connections. But it can also mean that DNS server is not configured correctly. Here are a few things you can try to connect to Yahoo.
1. Try accessing Yahoo using a different URL: mail.yahoo.com OR mg.mail.yahoo.com OR yahoo.com (then click on the Mail link)
2. Try using a different web browser. You can download Chrome or Firefox using the links below:
3. If it is a DNS issue coming from your computer's DNS settings and not your browser, try using a different computer on your home network.
4. If the DNS issue is with your ISP, try accessing Yahoo from a mobile device while using mobile data, instead of Wifi, or try accessing Yahoo via public Wifi at a coffee shop or Restaurant
Let me know which of these steps were able to get you into Yahoo Mail, and then I can help you fix the issue with your original device setup.