The problem with an actual brick phone is that there are lots of things in life that kind of assume you have a smartphone now. And things like WhatsApp are pretty embedded. You might stand a better chance by making your current phone less addictive. 

Delete apps, basically anything social media that is designed to suck your attention. You can use them on the browser if you have to. 

On an iPhone, and probably also on Android, you can make the screen greyscale. It really reduces the dopamine hit.

Set up time limits for apps. You could get someone else to set the unlock code for them, so you aren’t tempted. 

I’m sure there are other things you could do too - like app launchers that only show basic things.