Run a speed test on the same device and connection you use to watch, both during normal viewing and when the problem occurs. Compare results with heavy downloads paused. A stable connection matters as well as download speed; requirements vary by stream quality.
A speed test does not prove the cause of buffering. Low or unstable results can help investigate the network, but good results do not rule out a player, device, routing or stream issue. Compare Ethernet and Wi-Fi if possible, and tell support whether one channel or all content is affected.