Strange problem. Actually TP-LINK powerline adapter's performance has nothing to do with computers. So I suggest you should reset your WPA4220 and give a try.

You can directly connect your computer with your WPA4220 and use powerline scan tool. It should be working. If not, there must be something wrong with your product. In this situation, you can contact...

No, you don't need to do anything about your powerline products as TP-LINK powerline products are all pug and play which means zero configuration. Maybe there is no enough bandwidth in your powerline...

There is no firewall in TP-LINK powerline products. So you have no need to do any configuration on your powerline products. I think you can reset your powerline products to give a try.

A wireless powerline adapter will help you. You can check this product TL-WPA4220.

You can reboot your WR702 to give a try.