Choose reliability as well as resolution
A 4K edition can reveal fine detail on an appropriate screen, but resolution alone does not describe compression, color or the source material. A stable HD stream may be preferable to a higher setting that repeatedly stops.
Where to check the available quality
Open a title's viewing offer and read the edition and device requirements there. A sharp poster on VidSrc2 is browsing artwork, not a promise that the film is supplied at that resolution.
If the image keeps changing
Some players adjust quality as connection conditions change. Give the session a moment to settle. Where manual controls exist, compare settings without changing several other things at once. Heavy downloads on the same connection can also compete with playback.
For a large television, check the actual viewing app rather than assuming it offers the same quality as your phone. HDR, resolution and audio formats are separate choices. Device guidance can help you plan the setup.