As far as I know, we must have Ethernet connection to install the camera. But it is fine for us to use wifi after installing.

Maybe is it asking you create a new password? You can try to give it a private password. Or just do a reset on the camera and try again.

If the light lit up when it is connected to modem, then the ethernet port should be working. You may also check your PC's ethernet port by connecting the PC to the modem directly.

The issue is kind of strange. It might be the interference in the system. Not sure which device is interfering. If you disconnect the powerline unit from your router and keep it plugged in, will the...

I don't think there has such information, why do you want to know that?

Yes, the WAN IP address changes on the router, you can login your DDNS account on the router, then the DDNS domain name is binding to the WAN IP of your router. On the web interface of the camera, go...