i think my (secondary) question was misunderstood.
you answer the app question not being able to cast. thank you for that.
but you are incorrect about https://serverIP:32400/web being able to cast, as it now just redirects you to https://app.plex.tv first but it used to work nicely. i liked using the ip better because if the internet went down the prior link would still work, and as long as you didn’t pause too long the chromecast would continue to work too.
Yes you can. it will redirect you if you have not used it via that IP, so you can sign in then will redirect back to ip again. ( you need to use https not http )