Use Caddy for reverse proxy. It’s magic. Just put in config the subdomain/domain and localhost port to point to, it will fetch and configure and keep certificates up to date with zero effort. You’ll forget certificates exist. It just works.

One easy way to test accessing your site externally is use a free web proxy to try to load it.

For example (not a personal recommendation, just a random search result) https://wproxy.net/

Misleading headline wording. Makes it seem one thing lead to the other, rather than reality being the other way around.