Buffering is not caused by one single thing. Diagnose it in layers so you do not change several settings at once and lose track of the real cause.
1. Check the local network
Restart the router, test signal strength and compare Wi-Fi with Ethernet where possible.
2. Check the device
Restart the streaming device, close unused apps, install updates and make sure storage is not critically low.
3. Check the player
Update the player and compare behaviour with another compatible player if available.
4. Compare content and times
Note whether the problem affects everything or only one stream, and whether it changes at different times. That context makes support troubleshooting faster.