Sorry to hear this GiveMeStrength100. Think TickTackTock has some really good ideas to restrict phone/wifi use without removing the phone entirely- similarly your idea to have a basic phone that she could instead of iphone is a really good one.
I was a only child of a single parent, and one thing I do remember struggling with as a teen because that is a very intense of relationship. I particularly struggled with was being expected to be a sort of partner for my mum at times. This might not be the case in your situation of course, but I thought it worth mentioning in case it is making your DD feel a bit overwhelmed.
I struggled with it because I knew I would fail, as a teenage girl can't be a grown man/woman. I did try my best financially, emotionally and socially. And I was also at times a bit of a nightmare I am sure.
But it did kind of use me up a bit- I know my studies suffered, that other relationships suffered and that our long-term relationship suffered. In hindsight, I see that it was a really tough situation all round and we both did our best to plug the gaps and make choices that wouldn't put pressure on the other (she could have gotten married but both she and her bf had seen their kids struggle with the dynamics of a blended family with the former partner's situation, so in the end decided not to, I opted to work and contribute from 14 and also stayed at home to go to university when I could have gone away).
I also now see how the real pressure was put on us by a) my dad's actions and b) societies attitude to single parents. But at 14 I saw how my life was harder than a lot of my friends and I didn't really understand why, or what I had done to deserve that.
I also knew I had to pretend that everything was fine with friends and people at school otherwise I wouldn't be accepted, so I did what I thought those people expected for acceptance, well because school is social minefield as a teen at the best of times. This probably included bad behaviour at times. I was so desperate for social acceptance because of unprocessed hurt over my father's disappearance from my life (just the gap in attention that having only parent leaves) and also because I saw how hard it was for my mum to be a socially isolated single parent and I really didn't want to be socially isolated myself. So maybe your daughter plays nice with some people for social acceptance, and acts up alongside others for social acceptance.
And with me, all that confusion/resentment came out in the direction of my mum sometimes because 1) I trusted her not just to abandon me if I showed my real feelings 2) She was the one who was around most often so saw my confused and tired times the most and 3) I had a bit of a incoherent sense of it being my dad's fault and as he wasn't around my mum was the closest thing to my dad. As an adult, I realised that doesn't make sense, but being 14 isn;t the same as being an adult.
But with hindsight I do also see how some things could have gone better and how we could have found the intellectual and emotional friendship we found as adults a bit quicker.
So, if there is any element of co-dependency it might be helpful to separate out some of your expectations. I.e. 1. what is polite and respectful behaviour that you can expect from a teen (even though they are going through massive hormonal changes that rewrite their brain), 2. the social contact/emotional connection that is reasonable to expect from a child/teen, and 3. the social contact/emotional connection that a partner would provide.
Anyway, sorry that was long but I thought it might help.