infinite scroll for UICollection view

I'm trying to have unlimited scroll in my app where i'm calling the data from an API, and at least it works to some extent, but when i scroll in the collection view, it starts populating below and it cuts of the ones on top so i can't scroll back up to them, then after populating it just skips and goes out of range, here's some code please help.

var fetchMore = false
var pageNumber = 0

var productPageString: String {
    if pageNumber == 0 {
        return "pageNumber=0&"
    } else {
        return "pageNumber=\(pageNumber)&"
    }
}

func fetchProducts() {
    fetchMore = false
    let validatedTextString = UserDefaults.standard.object(forKey: "productsSearchValue")
    let searchText = validatedTextString!
    let urlString = APIConstants.baseurl + APIConstants.products + "?searchString=\(searchText)&\(productPageString)itemsPerPage=20"
    guard let url = URL(string: urlString) else { return }
    URLSession.shared.dataTask(with: url) { (data, response, error) in
        if error != nil {
            print(error!)
            return
        }
        do {
            let jsonData = try JSONDecoder().decode(ProductSearch.self, from: data!)

            self.productSearchResults = [Products]()

            for dictionary in jsonData.data {
                let product = Products(productId: dictionary.productId, categoryId: dictionary.categoryId, storeId: dictionary.storeId, name: dictionary.name, description: dictionary.description, imageUrl: dictionary.imageUrl, price: dictionary.price)
                self.productSearchResults?.append(product)
                self.fetchMore = true
            }

        } catch let jsonError {
            print(jsonError)
        }
        }.resume()
}

override func scrollViewDidScroll(_ scrollView: UIScrollView) {
    let offsetY = scrollView.contentOffset.y
    let contentHeight = scrollView.contentSize.height

    if offsetY > contentHeight - scrollView.frame.height {
        if fetchMore {
            fetchProducts()
            pageNumber += 1
        }
    }
}

It initially shows the 20 items per page, but as the page number increases, it keeps populating with the other content in that particular page, but i can only still view 20 items per page. i want to be able to scroll back up to the previous ones and also, why does it end up taking me to the end automatically? Thanks

It works now, all i had to do was remove the line

self.productSearchResults = [Products]()

it wasn't necessary