You really need to get back control of one or all of your online services (email, Instagram, etc.) Reset the passwords for all of them (including your the account to your cellular service), and not on any of your phones. Use a web browser of a PC/laptop or anything that's not one of your mobile devices. This will nullify all the previous auto-logins you had set up on your mobile devices so you'll have to re-authenticate yourself for each app involved since each will be tied to the previous password. For the services that support it, enable two-factor authentication.
It's really vital you get control over your own online presence and that involves your accounts online. It's your user account data and info that is stored online, your phones are simply accessing that data remotely. And your phones only retain bits and pieces of all that online account data. So it's not that every phone you get is being 'hacked' into by some remote exploit, it's a matter where your phones are getting compromised each time you set up one or more of those already compromised services. (i.e. if someone does have your password to your email service, it doesn't matter if you get another phone, or ten more phones. Each time you set up your email on another phone, that person will also get a foothold into that new phone. It's your online services you need to look into, and then work on securing your phone.)
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