Install from the App Store
Choose a well-reviewed player from the official App Store and check that it supports the format provided by your authorised service.
Keep Apple devices current
Update iOS, iPadOS or tvOS and confirm that date, time and network settings are correct.
Use a realistic trial
Test on the Apple device and internet connection you plan to use, especially during evening peak hours.
Improve playback
Close unused apps, restart the device and router, and use wired ethernet on Apple TV when available.