@Emanuel You might want to look at the settings of the place you are uploading the video to.
For example, when I export from my gopro at 4k, or the insta, it looks amazing on my phone, as well as on the TV on my home network. But when I upload it to youtube/vimeo/instagram/facebook, or send via whatsapp the video is downsized and down sampled resulting in the grainyness.
youtube for example will re-encode your videos as well which depending on size and format you have uploaded in, take a bit of time to do. Watching is straight after uploading will not look as good as hours or days later once the re-encode is complete.
And yes i have same suction cup mount as @Jaro Tuzinsky