This is the same problem as a number of other have reported, e.g.
Failed to fetch URL https://dl-ssl.google.com/android/repository/addons_list-1.xml, reason: Connect
New and real Solution! Tested On windows 10. 1- Download and run psiphon or freegate and connect it (turn on) 2- after, Search "Internet Properties" or "Internet Option"in start Internet Option Icon
3- then Open it and go to Connections tab then LAN setting and click on Advance 4- Now you can see the proxy address and port that psiphon or freegate uses it and Now copy those and past to SDK manager proxy Setting 5- then clear cashe and check all check boxes except "ask before restarting ADB" 6- now close the setting window and reload packages. congratulations, Now you have access to the all latest version of packages Notices! 1- do all steps, Step by step 2- connect psiphon or freegate before every thing and don't disconnect them untill you download packages you want! 3- if you disconnect psiphon every time, you must do all steps again. Good luck