Been a while and no one has a solution? I've setup my Archer c5 wifi 2.4 and wifi 5 both bands unticked for "isolation" but results were same. Sometimes devices connected on 2.4 would see the Chromecast and sometimes not. The reverse occurs too.

Finally I gave up on dual band totally and only activated one band and suddenly all Chromecast problems went away! You may want to give my solution a try ...