Presumably OP is using another remote? If so, they are not the first to lose volume control in the tvOS update. Experienced this with a friend's setup: all worked fine before the update, then no more volume control with the NOT-AppleTV remote that had historically worked fine with volume.
However, the simple solution is to use AppleTV, Remotes menu to re-learn the alt remote again. Teach AppleTV to recognize volume up and down on the ALT remote and it works again. Something in the update must have reset or forgotten the prior setup? Friends remote works fine again.