There are hints in your question that you might be undergoing some major rewrite/refactoring across the board. If so, that has always failed insofar as I've experienced or seen experienced it.
FWIW a better approach would have been to break the entire thing down into smaller, more immediately achievable milestones. Ideally do so alongside enough exciting new stuff to keep your developers' morale high - be weary that some might already have low morale if they're noticing that the project they're working on is going nowhere.
At any rate, the time to bring up the problem is likely now. Have a candid discussion with your team. Explain what's going on; collect data points you might be less aware of and new ideas as you do. Sketch out a brief but equally candid status of your current situation (in writing) to let your manager and the product owner in the loop, along with an outline of next steps get things back on track.
FWIW a better approach would have been to break the entire thing down into smaller, more immediately achievable milestones. Ideally do so alongside enough exciting new stuff to keep your developers' morale high - be weary that some might already have low morale if they're noticing that the project they're working on is going nowhere.
At any rate, the time to bring up the problem is likely now. Have a candid discussion with your team. Explain what's going on; collect data points you might be less aware of and new ideas as you do. Sketch out a brief but equally candid status of your current situation (in writing) to let your manager and the product owner in the loop, along with an outline of next steps get things back on track.