Unable to simultaneously delete emails on iPad and iPhone with POP mail

I can no longer delete emails from my iPad and iPhone simultaneously. It appears the POP mail setting prevents this. I have been advised the IMAP setting will allow you to delete emails from both iPhone or iPad at the same time. However, I cannot choose or change from POP to IMAP on either of my devices. This is new. I have always been able to delete one from the other. Any help? It doesn’t help if you have only one device with you. You can’t keep up on what you have read and already addressed. I’m sort of wondering if this started with the latest IOS update .