power button and volume buttons do not work

If your cable is properly seated, and you did not damage it during installation, then the h next thing that comes to mind and that you need to rule out it is the possibility that you have a defective cable; you'll need to try a new one.

Once you receive a new one from a different supplier if possible, test it before installing it: unplug the old cable, plug the new one instead, and carefully press/test the buttons. If it works, then remove the old one and install the new one. Don't go cheap with those cables. Choose the best quality you can get because they are such a pain to replace and you don't want to have to constantly replace defective parts. First because of the time you'll be wasting; second, because there is an inherent risk to every repair attempt, so you don't want to test your luck indefinitely. I recommend buying the iFixit cable because they usually carry the best possible quality and stand behind their product with a good warranty.