If the videos are starting and then buffering, it sounds like you might have a network issue.
Is the Mac mini connected to your local network via Wi-Fi or Ethernet..?
If you are using Wi-Fi, do the following and test.
1) Test the current speed of your internet connection.
2) Uninstall any and all third-party security software's and VPN's.
3) Restart both the Router and the Mac mini to reestablish that connection.
4) Check the Details of the Wi-Fi connection to make sure that "Low data mode" is off.
5) Temporally disconnect all USB devices to see if any of those are cause Wi-Fi Interference.
6) If it is possible, connect the Mac mini directly to the Router using a Cat 5e or better Ethernet cable.
Here are some items to help you troubleshoot the issue.
https://www.speedtest.net/
https://support.apple.com/en-us/102281
https://support.apple.com/en-us/101588
https://support.apple.com/en-us/102319
https://support.apple.com/en-us/102766