Setup a meeting with the dev team (only). Go over all the main features required for this release, ideally, they should already be in a form of "user stories".

Setup a meeting with the dev team (only). Go over all the main features required for this release, ideally, they should already be in a form of "user stories". Depends if you're in an Agile env or not... in 2018, I hope you guys are. Then during this meeting, run a deeper and more detailed estimate for each user story. How long would it take, what work has to be done and is it depending on any other team or future work? etc. When you get a better understanding and know how off the original estimation/deadline is (compared to this new one), setup another meeting with your PO and your manager to share this information. Obviously you will need material for this meeting. As a lead, that's your job to create that material (slides, etc..) That's it. As a lead, your job stops here. Any other decisions will have to be made by your PO or manager. Your role is to share that knowledge because these 2 don't have a clue about the actual work that has to be done down to the lines of code. Good luck (you're not doing anything wrong at the moment, just bubble up the information and let them do their job).