Hey there, is it possible that your internet speed may not be enough to support FHD on Disney+? Either way, this seems like an app issue, and is in no way related to your phone's performance.
As for YouTube HDR, the A53 only has a 1080p screen resolution. 2160p60 only works on screens that support 4K resolution :(
What's the other phone?
My guess is that it's an issue on the Disney+ app. I'm not sure if it lists FHD compatibility by device. If you can use FHD on other apps (which you should be able to do - the A53 does have a 1080p screen) then I think it's an app issue rather than a device problem.