I would try to focus on improving the communication and collaboration between the two teams. I'm on the backend team with only two of us. I'm the sole REST dev

I would try to focus on improving the communication and collaboration between the two teams. I'm on the backend team with only two of us. I'm the sole REST developer. The UI team has three people. I sit with one UI developer to my right, one right behind me, and the UX designer kiddy-corner. When features come up, ask five us talk about the requirements. The UI guys provide input on how they'd like the API to work. As we build, I ask them questions, they ask me questions. When I think I'm done, I push to a test server where they can hit on it. Sometimes they find I missed something or ask if I can add something. In short, were referred to in the company as the "app team". UI + API is really one team with the same goal. I would recommend coming up with ideas that you think would help move the project forward and present those to the management two levels up and try to get buy in from them. You might find yourself in an even better position if you can actually salvage the project rather than just leaving it.