It's working again, I rebooted everything again. Router, switches, phone, 271 and nothing changed.
I went into the app and deleted the 271 IP address and added it again (I had probably done this 20 times before) and this time it showed up under Device Manager (before it was just a grey box) but it could still not find it under Control despite several tries.
I then deleted and added it again and now it did find the 271.
None of this makes much sense.

Main: Marantz Cinema 50, Meridian 271, DSP7200.2, DSP5200HC, DSP5000 rears, Meridian 210.
PC/Headphones: Mytek Liberty II, Sansui CA-F1, Sansui B-2301, Sennheiser HD800, Kef R3