If I cancel the subscription. Can I sell my game and just pay the royalty fee without $19/month?
Yes, you are still be under the exact same terms whether you have a current subscription or not when it comes to royalty payments.
After your initial subscription, staying subscribed is just paying for access to updates to the engine and some marketplace content.
From the EULA
“After cancellation of your Subscription by either you or Epic, you will not be entitled to access or use future Versions of the Engine Code or future Assets that Epic makes available under the License. However, cancellation of your Subscription will not affect your rights under the License with respect to any Licensed Technology you have already downloaded under the License.”