Here are the steps. Go to 192.168.0.1 page of the router. Then go to Network-->Internet Access-->Change it to WAN only-->Save and reboot. Then connect the TP-LINK to the Cat 5 Ethernet cable form...

I think the signal strength 70db is better than 50db. How about change the angles of the two antennas? You can also use the Issider to check the signal or check if there is conflicts in your house.

As far as i know, if you want to use it as a AP, it is better to connect 4300 to your own router through one of the LAN port and disable the DHCP function, then there you go. It seems your setting is...

How about fixed channel to 1/6/11？Sometimes the security will also effect the speed. WPA2-PSK, AES is the best and most secured. Disable the Hardware NAT can be also a try.

It seems your homeplug is plug and paly. The same as TL-WPA271 does. It seems that they can work together, but i am not 100% sure. Is there anyone have the same model with you and have a field test?

It will take risk to use two different brands powerline. I think it is better to buy a TL-WAP271 kit to meet your requirement just in case the wireless extender can not work with your existing...