Speed is not the only factor
Required bandwidth depends on stream bitrate, resolution and codec. There is no single minimum speed that suits every stream. Even a good speed-test result can hide packet loss, variable latency or congested Wi-Fi. Consider what other devices on the same network are using.
Try Ethernet where possible
On a TV or TV box, Ethernet helps isolate wireless signal problems. If using Wi-Fi, check distance, walls and network congestion, and choose a supported frequency band. Change one setting at a time and retry the same content; changing many things at once makes the cause harder to identify.
What should you record when playback buffers?
Record the time, affected channel, device and app version. Pause large downloads, restart your device and router, and compare other content or another connection where possible. A problem on one channel alone may come from the source rather than your entire connection. Use official channels when sharing account information.