R7000 Nighthawk AC1900 home network via TP Link Powerlines

> My cable modem (virgin media super hub) is situated in upstairs and
> connects to my Nighthawk downstairs via a paired TP link powerline.

   If this works, then I'd guess that you connected a Powerline device
to the WAN ("Internet") port on the R7000.

   If you add more Powerline devices to that Powerline network, then
they'll be talking to the WAN port on the R7000, where they'll generally
be ignored by the R7000.

   Client devices must be connected to LAN ports on the R7000, not the
WAN port (which normally should be connected to only the modem).

   It should be fine to use Powerline devices to expand the LAN, but
that network must be isolated from the modem-router connection.  I
gather that this can be done:

      http://www.tp-link.com/us/faq-433.html

      Can several Powerline networks exist simultaneously in same power
      circuit?

      Yes.

      The powerline networks can be separated by different network
      names.  [...]