Usman, I had this exact same issue with Chromecast. Meraki is a little odd with it. What we ended up doing was set up an SSID specifically for Chromecast. We have the SSID running in Bridge Mode. We had to make sure in Wireless -> Firewall & Traffic Shaping to allow anything on Local LAN. After that, our devices were able to communicate with Chromecast and cast to it. I know you mentioned that it had nothing to do with wireless firewall but make sure to double check the SSID because Local LAN is denied by default. You have to change it to allow. Let me know how you do.
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