@gusgonnet I believe this is incorrect and in fact, the device must be connected to Particle for the claiming process to work. That is at least when using the cloud API and for a product. This was asked in this chat: Offline Claiming - #2 by rickkas7 where @rickkas7 gave a similar answer. I also tested this several times and IF the device is offline, then the cloud API response is: “{"ok": false, "errors”: [“Device is not connected to the cloud”]}. If the device is online, then it's successful. For now, I just flash a message back to the user to indicate they must power on the device first before trying to claim it. This will work for now but pretty sure I'll migrate to a single admin account within Particle that all devices are claimed to.
One thing of special note, is just because the device was recently powered off, doesn't mean it's "offline" yet, there must be some sort of timeout period before the particle cloud indicates offline (maybe the 23 minute keep alive?). For example, devices still show connected for a period of time after powering them off. This was the confusing part for me in my initial testing as I didn't wait long enough for the particle cloud to indicate it was truly offline like the last device ID in this picture of a gray dot vs the breathing cyan within the console.
Not really sure why this is a limitation. I don't think the device itself knows who it's claimed to so why does it have to be connected for this? Maybe it does. Maybe this is by design and if by design, I'd be interested in hearing what the rationale is/was when implemented. All more of a reason I think to have a single admin account claim all devices for cellular products (i.e. able to subscribe to events since it is claimed, and able to do offline claiming since that is only linked in our own backend)