Trouble connecting to the internet

Assalam o Alikum

My name is Awab Khurram and I was having serious trouble with my laptop. I recently downgraded from windows 10 to 8.1 because it was lagging and all of those problem i did a clean install but not when I booted in to my computer i noticed one thing that, what ever I do the computer is not browsing or executing any apps which requires the use of the internet, I tried everything like my drivers, network adapters but doesn't work. I tried with a microsoft tech nut no use they say that contact your ISP but they say check your windows. The problem is I am an undergraduate i am in high school but i am also training for Microsoft professional exam. I really have less experience in my field and i tried everything i learned but still not working. Please help