I couldn't get the kaminari-cells gem to work with Rails 6, so I copied these two files from the gem into my project and made some slight changes.

cells.rb

# app/models/concerns/kaminary/cells.rb
module Kaminari
  module Cells
    extend ActiveSupport::Concern

    included do
      include Kaminari::Helpers::CellsHelper
    end
  end
end

cells_helper.rb

# app/helpers/kaminary/helpers/cells_helper.rb
require 'kaminari/helpers/helper_methods'
require 'cell/partial'

module Kaminari
  module Helpers
    module CellsHelper
      include Kaminari::Helpers::HelperMethods
      include ActionView::Helpers::OutputSafetyHelper
      include ActionView::Helpers::TranslationHelper
      include Cell::ViewModel::Partial

      def paginate(scope, paginator_class: Kaminari::Helpers::Paginator, template: nil, **options)
        options = options.reverse_merge(:views_prefix => "../views/")
        super
      end
    end
  end
end