Is it being a NAT router (and DHCP server), rather than a bridge?
I think, and I'm not exactly red hot on this stuff, it was being a bridge. I base that upon the observation that the clients mac addresses were listed in my routers DHCP client list and as internet traffic sessions in their own right.
Your not being dumb. A power-line Ethernet adaptor even with WiFi should be a layer2 device so it shouldn't matter what you set its IP address to anything connected to it should still be able to DHCP. Of course sometimes logic doesn't come into it with firmware sometimes it just does strange things and you can never find out why unless you have and can read the source code.
Why not leave it all on DHCP and statically reserve IP addresses for the adaptor on your DHCP server ?
Tried that too. It worked in that the end points accepted the non-pool assignment and clients got pool addresses but it was very, very flakey. I tried 4 clients, a pi and an elderly Kobo slab could get IP assignments but actual traffic was rare. Big river slab and my phone - just an endless cycle of 'connecting...'.
As mentioned
in another thread (I'd forgotten about this post) Devolo support confirmed my desired usage was hopeless so they went back. Ideally (for niceness of solution but not for wallet) I'd just have a trunk bridge over powerline and stick a DrayTek AP out there but that requires VLAN support. There's one powerline vendor in Trumpistan that supports VLANs but none of the players in the UK do. OK, I didn't check them all but I emailed TP-Link, TrendNET and Netgear plus found the specific question online for D-Link. They're all 802.3ab.
I have a much cheaper Asus pair waiting to be set up, the 2nd remote end point can wait. Interesingly, you can read the source code for Asus gear. And I do mean you. While I'm sure I could reach each line and understand what that line is doing there's no hope of me being able to grok the bigger picture.
ETA: Found it,
Nexuslink is the vendor that supports 802.1q and does so despite being only 802.3ab not ac.