I don’t have specific experience with the gen7 series, but firmware updates ostensibly come as an ISO or USB image which you can boot in lieu of your normal OS to apply firmware updates. At least, that’s one of the ways I think HP would still support, in case customers are running neither Windows nor a Linux-based OS.
To rule out a cable-specific electrical issue at boot, what happens if you boot the server with the cables attached to the controller, but not attached to the d2700?