Issue with self-assigned UP Address

[#1]

Hello,

I'm using a PowerLine adaptor and ethernet cable, with intermittent success. Most of the time it doesn't work, and says "Belkin USB-C LAN has a self-assigned IP address and will not be able to connect to the internet." 

I've tried changing the IP address manually, but that hasn't worked. It says it's then connected, but no web pages load. Tried the fix here but did not work for me: https://community.plus.net/t5/My-Router/Self-assigned-IP-address-on-ethernet-cable-with-plusnet-hub-...

Any ideas?

Thanks

[#2]

Which one of the power line adapters doesn't get the IP address.
The one plugged into the wall and router or the one that is getting the network from the power cables?