You need as much fan as possible. If there were a 200% setting then you would want to use that. By default fan comes on very slowly and not full fan until 5mm or so. If you make the fan go from off to 100% on the second layer you will get underextrusion because the head will cool to fast and it takes 30 seconds for the PID temperature controller to compensate. So I recommend having the fan come on even for the first layer. You will need a warm bed (70C) if you print first layer with fan on.
This will help a lot but it won't be perfect. It will be better.
OR FLIP IT OVER
I don't know any way to get this perfect. I guess you would have to flip it over - that would make good curves but the hidden side will be a bit stringy. You will want 100% fan for that also when it bridges the back of the phone. You will need at least 4 layers to get the top layer perfect as each layer will get better (they will all be a bit low and slightly underextruded but each layer will get better).
Printing fillets like that can be problematic since it becomes such a big overhang. What I'd try to improve on it would be to print thinner layers (0.1mm), reduce the temperature of your bed to 60C and try turning the fans on sooner.
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