PowerApps can manage only 2000 items max
If your SP is over 2k yo will need to create Collections in PowerApps to collect the data and the create the master collection to join all the values
In the On Start option or On Visble (of your first sceen)
insert this
ClearCollect(
Collection_1,
Filter(
SP_List,
ID < 2000
)
);
ClearCollect(
Collection_2,
Filter(
SP_List,
ID > 2000 && ID < 4000
)
);
ClearCollect(
Master,
Collection_1,
Collection_2
) //Your will need to use a differente ID column the ID from SP wont work for this formula
Then in your dropdown use your formula
Sort(Distinct(Master, CommunityName), Result)
