You can't run unsigned apps on an iPhone. No, not even your own.
You can't generate a certificate for an iPhone app longer than one week, and even then, the certificate you generate only lets you run code on an iPhone physically connected to the computer where the certificate was generated.
EVERY OTHER way to get a certificate, including for internal development and testing through testflight, requires the app to go through review. Every. Single. One.
It's not simply for putting the app on the app store, it's for running the app at all.
Enterprise certificates were supposed to be a way for a company to avoid shipping their internal tools to Apple, but they turn out to be a way to actually run useful code on an iPhone, even if it's not something Apple wants you to do.