Apple TV on fire stick

Please consider your DNS settings. It is mostly recommended to have the devices get their DNS from the router, and the router should get it from the ISP. Possibly a router restart can clear some caches, negating the issue. Custom different settings might cause issues in some cases, as those might not be as up-to-date, or filter/block certain connections, or other things outside the control of the app or the remote server with the content.

Recommended settings for Wi-Fi routers and access points - Apple Support

Else contact the Support staff for further assistance.

Have you tried restarting the Fire TV device? Restart Your Fire TV Device - Amazon Help & Customer Service

Have you tried checking for system software updates, and installing that when available? Update the Software on Your Fire TV - Amazon Help & Customer Service