how I render clear video for mobile

There is a possibility that the video has a smaller resolution than your iPhone has. What resolution is your original clip and in what resolution are you exporting? Let's start there to get started.

Use the Adobe Media Encoder to render an H.264 MP4 using one of the YouTube or Vimeo presets that match the frame size of your project. Your composition should also be created using one of the presets that use Square Pixels. Your comp should be HD (1920 X 1080) at the bare minimum for any mobile device and 1280 X 720 if you plan to share on Zoom.  The frame rates should match the region you live in. PAL is 25 FPS because PAL countries use 50 Hz power and NTSC Countries use 29.97, 23.976, 24, or 30 fps as standards because the countries power grid runs at 60Hz. If you want to create custom frame sizes, frame rates, custom aspect ratios, and custom compression settings and you are not an expert in video standards and compression technology you are asking for trouble. If you are uploading your video to any video sharing platform you will have to use their standards and wait for them to process the video before the full resolution version is ready for playback.

Since you haven't offered any exact specs for the video we can't tell you much, but it seems to me that you are ignorant of the fact that pretty much any online service will re-compress your video and generate multiple versions of it to target specific platforms. So if you upload your video to Facebook, Twitter, Instagram, YouTube and so on you never get to see the version you actually created. this has nothing to do with AE. It's simply how these things work.

Mylenium