How to perform a diagnostic test on iPhone?

You can't do this, you'll have to have to tell Apple about the issue(s) you're experiencing and then they may run diagnostics on your iPhone.

You can contact Apple Support here: iPhone - Official Apple Support

Some assumptions: If you’re really still running iOS 14 on an iPhone 11 per the footer, you’re probably operating near or at available storage capacity, and filling an iPhone full can sometimes cause issues. Performing a backup, factory reset, and restore can usually resolve any latent storage corruptions. It’ll also get the most current iOS. But a caution: everything might not fit back onto the iPhone, if storage is already tight.