Hi,
Normally we do not help with such cases, cause we not know how and why Chromecast says that, so I had to do some research and found maybe some possible ideas that may help or not:
1. Make sure that your wireless AP has separate clients feature turned off, when you go to your wireless configuration, click on Edit next to your AP and enter Advanced Settings, there you will find a tickbox Separate clients, which should be disabled.
2. You could also try swapping encryptions and ciphering methods of your AP to different ones.
3. What is your main WAN setup on RUT950? Wired, mobile, or station mode? You could try swapping between those and see if it does anything.
4. Try asking in Chromecast forums, maybe there will be more answers on how to workaround this thing, because if your router is not sharing a hotspot AP - then this shouldn't be the issue of the router itself.
EB.