Any anti-virus software running on your PC? Hard reset your router and try on different PCs to set it up.

If C20i works as the main router, you should confirm if the old router has WDS function or not and you should configure it on the old router.

The ipad is on the third floor while the main router is on the first floor? If so, I don't think range extender is the best solution for you to extend Wi-Fi network. It would be much better to use...

You want to block devices in the list to access none of websites during a period of time, right? If so, to use parental control rule, in Restriction policy, try to add a invalid url like...

If you don't need Wi-Fi connection to powerline adapter, it is okay to use PA4015PKIT. But if you need to connect powerline adapter wirelessly, you need a powerline extender to work with the main...

This is TP-LINK forum. You didn't realize it?