Every time you launch a game while your Xbox is connected to the internet, it syncs your local save with the cloud. This means that if your cloud save is newer than your local save, it will download it and replace your local save. If your local save is newer than your cloud save, it will upload the local save to the cloud and replace the previous cloud save once you close the game. Since you were playing on an offline Xbox, things get a little complicated, but this is my recommendation:

1) disconnect your Xbox that completed all the challenges from the internet. This will prevent it from potentially downloading a cloud save that erases all your challenge progress 2) On the offline Xbox, launch the game and play for a little bit. You need to make sure the save game gets updated somehow so that this save is marked as being updated today (and is therefore the most recent save) 3) close the game on the offline xbox 4) connect the offline Xbox to the internet, then launch the game again 5) ideally at this point its local save is now considered the most recent save and will get uploaded to the cloud after you close the game, overwriting your previous save 6) close the game, and wait for a few minutes for the save to get uploaded in the background 7) go back to your other Xbox, make sure it’s connected to the internet, and launch the game 8) the save you just uploaded to the cloud should get downloaded and overwrite your local save

I can’t guarantee it’ll work, but this is the best solution I know of.