It's not a problem to look down to your phone as long as the back of your neck is straight.

try these 4 things in combinations:

1. Rotate pelvis, it's the same as sticking your butt out and your crotch area should point to the floor, not forward. This will cause a sway in your back though thats when you add in point 2 to correct the sway in your spine.

2. Contract your upper abs, don't tense them but suck them in and then hold them. This will cause your ribcage to rotate the same way as your pelvis and removing the sway in your lower back.

3. Keep your shoulders back but not too far back, just so that they're not rounded and so that the veins running from your chest through the shoulder to the arms are not restricted by the rounded shoulder.

4. You'll notice that each step makes it easier to do the other step. So the last step is the neck straight above the shoulders, when you do this you'll notice that your head moves perfectly the way we want it. Now you can look down on your phone as much as you want.  (it'll be awkward at first and you'll have a double chin, but that's temporarily it'll go away once your mandible angle improves and moves forward)

5. and then as a finisher mewing, ofcourse.

when you're relaxing or standing try to put your body weight on your heels, not your toes.

EDIT:

When you're breathing keep the upper abs like that too, because this will make you breath more from your chest rather than your stomach. It'll give more room to the organs below the ribcage over time, because your breathing moves you up and down (Ribcage) rather than in and out with your stomach.