Upgrade to an unsigned version of iOS - possible or not? 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?
No. It is not possible to installed an unsigned version of iOS.

The only version you can upgrade your iPhone 5s to is the current latest available for the device which is iOS 12.2.

If you have the source code for the app that you wish to test on iOS 11, you can test the same by building it for iPhone 5s Simulator running iOS 11 (you can test the software based features but not all of the hardware based features this way).