It looks like your organization has some fundamental problems.
I'd have a high-level chat with the person defining the roles and competencies and more importantly: the meaning of UX.
You're a UX Designer, but you don't do UI and you don't have anything to say about it.
TLDR: UX Designers are highly responsible for the UI too.
"User experience" encompasses all aspects of the end-user's interaction with the company, its services, and its products. - Don Norman, the first human to define UX
If you "don't do UI", you're a cook who isn't concerned with cooking... You researched your ingredients & customers, you wrote your menu and then you hand it over to someone who'll heat it up for you and smack it on a plate without knowing if it's gonna be served out to a posh business man or a road trucker on the go.
The UI Designer ignores your research.
TLDR: either your research is incorrect, or the UI Designer is
ignorant. Try convincing him with visual methods like an infographic.
Since you didn't share any information about the research, I can't tell you that the UI Designer is wrong. But I can tell you that if you're right (fair chance on that, since the argument of the UI Designer isn't valid, check the answer from "Arriving from the sky"), you could've perhaps done a better job explaining and advocating the research.
To convince people, I'd advise following communication courses. Those can teach you the golden circle, Neuro-Linguistic programming, Myers-Briggs Type Indicator, etc. Or presenting your User Research as an infographic can help the visual people to understand it better.
Also, give your UI Designer SBI feedback about this.
As a last resort if he doesn't listen, tell your manager / product owner / lead designer he's obstructing business potential. But do it respectfully.