Firebase firestore snapshot with limit and pagination

[DEVs]

I am working with Firestore right now and have a little bit of a problem with pagination.
Basically, I have a collection (assume 10 items) where each item has some data and a timestamp.
and First 10 items get with snapshot orderBy(createdAt ,desc) after that I take one variable as a last record and i used it in another query as a startAfter clouse but when I add new record then first item is removed
if we have one array in that we have 1 to 50 items then on the first time i get 10 item with snapshot like 1 to 10 and If I add new item then 1 is remove and new item is pushed in array and if I clicked on load more button then in the list i saw 1 is removed
how can I solve this problem?
EX.
In firebase collection Array = [2,3,4,1,8,7,6,9,5,10,13,12,14,11,16,17,18,20,15,19]
first snapshot with order by createdAt desc and limit 5 I get and I also use docChange() method in snapshot
16
17
18
19
20
after i clciked on Load More this is get() method same as above limit ,orderBy and startAfter then I get
11
12
13
.
.
.
20
if i add 21 then I get
11
12
13
14
15
17
18
19
20
21
after reload I get it back in perfect order
My Tasks are:
1.Delete items from array with real time update
2.Edit Items with real time update
3.At the first time I only want to use Snapshot with limit 10
4.After than other record get with simple firestore get() query