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I have an iPhone 5s (A1453). It is currently running iOS 9.x.
I need to upgrade it to iOS 11.x to perform testing of apps on this particular iOS version/device combination.
I've read couple of articles about a method with Prometheus, SHSH2, etc. But it seems that I'm out of luck, because this device never had iOS 11 installed on it, so there's no SHSH2 for this version, and it is already unsigned.
Am I right with my guess, or is there some way to do such an upgrade?