How to use firebase messaging without valid SSL certificate

I'm using google firebase Messaging for push notifications of my website. my website doesn't have a domain. Its using HTTPS with self-signed certificate. as i realized service worker needs HTTPS with valid SSL certificate. witch isn't available for sites without domain.
in some of other push notification services there is an option for HTTP websites that it opens a new window with a sub domain and notifications will be received from there.
is there any free option in any site that provides such future?

or

is there another option for me to pass invalid SSL error when enabling service worker?
btw: i'm aware about browsers flags for allowing self-signed certificate. i'm not gonna use that because i have some users now.

my frontend is a angular cli app running with nginx on a ubuntu server.