That isn't how dopamine works. If you have ADHD you require more stimulating activities in order to get a satisfying response basically. I don't think a feature phone will fix the problem. Possibly gamifying your ability to not go to those things on your smartphone might be a better plan. Figure out the situations in which you pull out your phone and try to be mindful of those situations and change The situation or your response to it.

You said you went to keep your iPhone at home and put your SIM card in a dumb phone. So does that mean you want to take your phone to work or when you go out? What were the situations in which you pulled out your iPhone and you mindlessly would browse the internet? Are you a coder and that was what you would do when you would wait for a compile or something? Any kind of downtime you would pull out your iPhone like a cigarette smoker would take a smoke break? Try to figure out those situations and come up with alternate activities to do. Once you figure out those alternate activities try to make those alternate activities have the lowest barrier to entry. Obviously pulling your iPhone out of your pocket or inches away from your desk is a whole lot easier than getting up and taking a break and taking a walk for 15 minutes or whatever.