@Jbroni 

From  the powerline rate,  the actual speed is pretty slow.

When Router < Ethernet cable #1  >PA411......... PA411  < Ethernet cable #2  > PC: 

• Make sure both Ethernet cables are cat5or above, should support 100Mbps
• Check the wired link speed between PA411 and PC , it should 100Mbps, not 10Mbps.

 (How to check the associated/link speed on a wired client)  https://www.tp-link.com/support/faq/2899/

• Check the if router's LAN port is on 100Mbps if you could check the port speed on router interface * Try a newer a cable between router and main adapter
• Try a different wall socket for the extender unit or main unit one by one and see if that helps

May I know when router --PC , how much speed you could get? If locate PA411 closer in the same room, will the speed improves, when did you buy the KIT and did it work fine before?