Assuming the redis app and vm app are in the same organization, they should be able to connect using .internal, using the port (which would be 6379, so it’s not that) …
Are you using the Fly image for Redis (Upstash for Redis®* · Fly Docs)? Or your own? Only the Fly one requires a password so if you aren’t sending one, that’s an easy thing to miss.
Next thought: is this container a Node app? Only that can have issues with IPv6 which is not the default. The fix for that:
Ruling those out as the cause … er, that’s the extent of my knowledge. Someone else will be able to assist though.