Check your own network: run speedtest, make sure that the 4K channel receives at least 20 Mbps, connect the device to 5 GHz or via Ethernet.
Change DNS to 1.1.1.1 or 8.8.8.8, disable VPN/Smart DNS and temporarily turn off the antivirus on your PC to prevent filtering.
If the problem repeats at the same time, remove the logs (on Android TV in TiviMate → Settings → Logs → Export) and send to the provider along with the channel name and time. This will help the team find an overloaded server or a downed CDN node.