how to add lastmod field in index sitemap django

I need to add lastmod attribute to sitemap index. And it looks like django.contrib.sitemaps.views.index doesn't include lastmode though Here is my sitemap.xml:

<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
<sitemap>
<loc>localhost:8000/sitemap-pages.xml</loc>
</sitemap>
</sitemapindex>

my urls.py

sitemaps_pages = {

    'pages': sitemaps.PageViewSitemap,
    'life': sitemaps.LifeViewSitemap,
    'lifes': sitemaps.LifesSitemap,
    'novosti': sitemaps.NewsViewSitemap,
    'novost': sitemaps.NewsSitemap,
    'catalog': sitemaps.CatalogViewSitemap,
    'categories': sitemaps.CategorySitemap,
    'regions': sitemaps.RegionSitemap,
    'times': sitemaps.TimeSitemap,
    'material': sitemaps.MaterialSitemap,
    'products': sitemaps.ProductsSitemap,
}
    path('sitemap-<section>.xml', sitemap, {'sitemaps': sitemaps_pages}, name='django.contrib.sitemaps.views.sitemap'),
    path('sitemap.xml', index, {'sitemaps': sitemaps_pages}, name='django.contrib.sitemaps.views.sitemap'),

sitemaps.py

class ProductsSitemap(sitemaps.Sitemap):
    protocol = 'https'
    try:
        priority_filter = strip_tags(Info.objects.get(name='priority_filter').value)
        frequency_filter = strip_tags(Info.objects.get(name='frequency_filter').value)
    except Exception:

        priority_filter = '0.5'
        frequency_filter = 'daily'
    priority = priority_filter
    changefreq = frequency_filter
    limit = 1000

    def items(self):
        return Product.objects.all()

    def location(self, item):
        return str(item.url)

    def lastmod(self, item):
        if item.url == '/':
            return Product.objects.latest('updated_at').updated_at
        else:
            return item.updated_at

How do i solve the problem?