Quick glance at the code, you should add more debug(print()) code to see what is going on at most functions to see where the second player is at through the whole joining process. I didn't go through your whole code base, so this is just general helpful information.
I watched the video and you are clicking the host button not the join button as well. I assume this is built to be the main player hosts a server and everyone else joins. If so you need to make sure that you hosting/ joining properly or that the code is working around that as well.