Check if the stream requires specific headers or user-agents. MPD streams often utilize DRM (Digital Rights Management) like Widevine or FairPlay. If the stream is encrypted, your video player must be configured with the correct license server keys to play it.
Free public M3U lists found online often have user limits. When too many people access the same M3U8 chunk simultaneously, the host server drops connections. Private, dedicated streams avoid this issue. video player mpd m3u8 m3u epg link