Ask HN: When could or should a blockchain be used as a back end for a mobile app?

Not sure what you mean, but just in case : there is this blockchain based mobile cryptocurrency called Electroneum. They plan to release a mobile mining simulator on mobile. Check their website.

Almost never ever.

What are you even trying to achieve?

Blockchain should only be used when adhering to a hype driven development paradigm.

can a mobile phone have a static IP?

don't you need that to host a blockchain?