It might be easiest to get a domain name. You can get a domain name in the .xyz top-level domain of six to nine digits (e.g., 123456789.xyz) for about a dollar per year. I’ve used Porkbun as my domain name registrar for several years.
Alternatively, I’d also consider using YunoHost. It’s how I got started with self-hosting a few years ago, and @fflorent has packaged SilverBullet for YunoHost. You can set up YunoHost with a free subdomain, and it’ll largely handle DNS and HTTPS for you.