From the relevant law:
If by his actions and positioning it is apparent to the umpires that he will not be able to discharge his duties as a wicket-keeper, he shall forfeit this right [to gloves and external leg guards] and also the right to be recognised as a wicket-keeper for the purposes of [other laws].
I haven't seen the incident, but the umpire evidently opined that standing at the 30-yard circle is not a suitable position to discharge the duties as a wicket-keeper. Which is probably reasonable, because the fielding side was probably trying to have an extra "normal" fielder to stop runs rather than someone tending to the wicket. So the umpire advised the player to either come closer to a position suitable for a wicket-keeper, or he would have to take off his gloves and leg guards. The alternative would be that the umpire does not advise the fielder and immediately calls him for illegal fielding, but that's not how things are typically done.