Unable to turn on data saver or switch data plan on Samsung s10

We have 4 phones on our plan that shares 10 gig data. We are able to turn on data saver on all, but Samsung s10. it is using  a lot of data. We tried to switch that phone to unlimited data plan, but keep getting errors. Unable to do so. other phones do not have the problem, except Samsung s10. I tried mobile app as well as xfinity website.