Just been through similar diagnosis. Mesh network working fine, then when one of the Satellites went to flashing red led, started investigating.
Summary is that the ORBI router updated firmware and propagated that update to the satellites. Even though the affected satellite was at that time in the same room as the Router with a clear connection, something 'went wrong' with the firmware update on the Satellite. The net result is that Satellite is now completely bricked - the red flashing light means the firmware is corrupted and Netgear have not built any mechanism in to the Satellites to revert back to a known good firmware.
The router you might be able to deal with if you can get to the Ethernet port (use the TFTP access if the internal OS can get as far as enabling the port to put a good firmware file in the right place). Unless you get lucky with a Shock Reset (reset hold for 30, still hold and power off, hold for 30 more, still hold and power on, hold for 30 more, release), any satellite is bricked; if you can't manufacture a way to the Ethernet port and a shock reset doesn't work, the router will be bricked. If it is in warranty, get it replaced. If like me you bought the new, unopened, fully boxed package from someone Netgear doesn't directly authorize, then there is no warranty and the item needs to be thrown in the bin with minimum of crying (returns not an option when it was working on receipt and Netgear do not take responsibility for the ORBI firmware update process killing your kit).
Moral of this story is, for the RBR/RBS10 models at least, unless you know that the firmware update is required, don't risk it. I'm an IT professional with 25 years experience working with, among other things, network components, and fairly familiar with this stuff. This was simply a case of a poor firmware update process not being safely implemented and not being recoverable. It's a weak design in these components and avoiding firmware updates is the only option. I have three satellites left - I don't know what the attrition rate will be but I'll be taking firmware updates for known security issues only now, and looking at alternatives if I lose another satellite.
