How to Avoid Banding and Noise?

Hello !

I've been getting trouble with Banding and weird noise in my shadows with different plattaforms. (Especially Instagram)

If I export, mostly seems fine in my computer. But I don't know how to avoid the banding on this platafform.

In other videos I always look the shadows clean. Here some pics for comparasion.

First from my computer:

The second from the post:

This is driving me crazy. Can someone help me?

I try different options like export in different depth, and nothing.

Mostly my reel I use VBR 2 between 15Mbps and 20.

PS: Sorry for my bad english, I'm from Guatemala.