Picture quality depends on source quality, bitrate, decoding, display settings and network delivery. A resolution label or a 4K television does not prove an individual stream contains 4K detail.
Check the source before the screen
Compare another item and inspect the source information the player actually exposes. A channel name containing HD or 4K is not an independent measurement. Compression, motion and source quality can differ at the same stated resolution.
Adjust one setting at a time
Check aspect ratio and any player output setting without stretching the image. If a decoder option is available, compare it on the same item and restore it if playback becomes worse. Keep TV picture settings separate from player settings in your notes.
When quality drops during playback
Compare actual delivery under household load. Do not infer the cause from one speed test. If other items remain stable, ask support about the affected source rather than changing every display setting.