At that point, it might be too late. It's hard to reset expectations in the middle of a project that's already gone off the rails, short of pulling the team and replacing them with a client relations person, hence my advice.

If OP is "just" an engineer, I feel like they've still neglected their responsibilities as a consultant, and I think all my advice stands, because it means they didn't talk with their manager about this project, they just went in blind, full of assumptions.

At the consulting company I mentioned, everyone had to go through consulting craft 101, everyone had to learn how to be "client-facing," because at any point someone from the client might want you to add something out-of-scope that should be just five minutes and you might want to do it to be nice and make friends.

I have absolutely had loud, difficult arguments with technical leads and project managers who did not do all the due diligence necessary to inform my scope of work. That's my responsibility to make sure I'm able to execute, I'm set up for success, and the project doesn't fail.