I've run into this issue twice now, both within start ups, the first where product development was directed by marketing alone, the second where new products and features were dictated by sales alone and development was owned by engineering alone.
It's always a problem. You are not alone. There are solutions.
The biggest challenge is perspective shift, namely moving from one team/person dictating direction to the team agreeing on everything getting done. This is where buy in from all stakeholders becomes critical. How to do that is actually pretty easy: go to everyone individually on a given design direction and decision and, one by one, use data and research to win them over. You'll find this process awfully rewarding, both because of three challenge of argumentation and because it is inherently post is discovery too. Continue doing this until you get everyone to agree...not most people, everyone. If you don't that opens the door to disagreement, and at the risk of sounding political, early on that can be dangerous, at least until the various people/teams are working together and trust each other.
Otherwise, do your job as a UX designer. Gather data and test assumptions and price hypotheses. Show your work and evidence from your work. Don't do it alone, bring the team in, get feedback from within as well. The end goal isn't to really the company behind you or design, it's too unify the team and to build trust. Once that's done, UX just needs to fulfill on those resume bullet points.