If a guest gets stuck in the media check, it's because one of the devices failed.
There are two simple solutions for the guest to enter the meeting anyway.
1. Enter the meeting with the failed camera/microphone/speakers.
When entering without a microphone or speakers, use the dial-in option to establish an audio connection.
2. Try to troubleshoot the problem.
If a device is failed an automatic troubleshoot will appear in the chat. The guest has to simply follow the steps that are shown to him. You as an advisor can follow the suggested troubleshoot options and the replies the guest gives. This way you can follow the troubleshoot in real-time.