It really depends on the platforms you want to support. The Samsung J7V is a 3 year old “low spec” device, and it will have troubles running 4K videos. If this is a device you want to support, you’ll need to use HD videos rather than 4K.
The iPhone XR is a highly capable device, which we test with, so I’m really not sure how you would see the issue there. Can you export your content and send a link to it to firstname.lastname@example.org? We could test it here to see if we can see the issue.