Over the last day or so I've noticed my Chromecasts Gen1, 2 & 3 now all show an empty rectangle instead of the wifi signal strength. Router powered off and back on to rule that out. Someone on the chromecast subreddit also reported the same issue: https://www.reddit.com/r/Chromecast/comments/zbkmgf/chromecast_gen3_shows_a_square_box_instead_of/
Just checking in to make sure that you've seen my response. Please let me know if you still need help or if you have other questions or concerns as I will be locking this in 24 hours. Feel free to start a new thread and we'll be happy to help.