My mid 2010 High Sierra is not functioning

Okay so basically I have had this problem for the past three weeks and I can’t seem to solve it. Basically my iMac is not working properly and it’s giving me the same message every single time saying “The recovery server could not be contacted.” So I tried MANY ways to fix this problem but it wouldn’t work. I have tried to change my Wi-Fi connection and it still gave me the same error message as before. And there is this one disk called KINGSTON Media which I thought would be the problem because the storage on the disk was full. And also the fact that my iMac sent me from 10.13.6 High Sierra to freaking 10.7 macOS Lion made me super upset because nothing on there is supported. All I want to know is how can I really give it a hard factory reset and return back to normal. I also wanted to upgrade it to 10.15 so then I can do more things. Anyways Apple, if you can help then thanks a lot