Hi @tahsu ---- Thank you for touching base with me and sharing the observations made during your troubleshooting.
As mentioned, while I am not personally familiar with the how the Devolo homeplugs should be configured, I believe this is the place to start troubleshooting your issue. I did a quick search on the Devolo website to see what I could come up with, but could not find anything in their “FAQ” section related to your issue 
Have you tried contacting their support team yet?
Furthermore, as a general rule, we recommend making use of a mesh network setup. Companies like Erro or Orbi, seem to be very reliable. Our CTO Brian, goes into further detail here in regard to the advantages offered by both a good quality AC router or “mesh setup”.
-Eric