Helo,
I have a Rust server on the port 28015 , is it possible to protect that port ?
I think that I protect the port so that some countries cannot enter through it: for example, China, Taiwan,Russia, Tayland.
I know that in fact it should be done through the server firewall but I can't find any specific tutorial for this.
Is there a plugin for this thing?

Maybe this plugin can help you? https://umod.org/plugins/countries
"Limits players connecting from certain countries "

I meant on the server level, not the game lavel.

In that case I do not think there is a plugin for it.
If your server is linux based, look for iptables in combination with the geoip module.