@Helenl 

If House Leds are ON on power line units , you could try some suggestions here:
1> change a different Ethernet cable between Main router and main power line adapter
2> try a different LAN port on main router. 

If still the same problem,please connect a computer to router by Ethernet cable ( same cable that main powerline adapter is using) and check do you have any Internet? Please send us a photo of the IP address on PC when wired to router. 

How many Leds are ON  on the main adapter and powerline extender? 
If connected to AV600  by WiFi or by wire, what is the IP address & Gateway on your computer? https://www.tp-link.com/support/faq/838/

Thank you.