Hi hope someone can help

i have five deco m5's around my house and wanted to improve signal between two so bought tp link power line adapters to use! Problem is when I connect the Ethernet cable to the deco the led goes red and I lose connection! Removing connection and deco resets! However if I just get a long Ethernet cable between the two the system works fine but unsightly wires between the two rooms??

any idea what's happening and how to resolve please?