Why is it that when the asterisk server crashes, users still have the ability to connect to each other?

Question

Asterisk

Where I screwed up, it seems that it should not be so. At least when I used freepbx this was not the case. I also noticed that the online / offline statuses are displayed incorrectly, as if always online. How to make it so that as soon as the server is turned off, the clients display that there is no connection to the server?
Asterisk acts as a sip server, located on a public ip, the port is standard. The server is being brute-attacked while only fail2ban is configured, but there are plans to change the port or hide it behind a vpn. Clients currently do not have a public ip, they are behind nat. Suspicion of the curvature of softphones, and the lack of configuration in sip.conf

Answer 1

I read about nat in this article, I have not tried it yet, because. time is 5 am, but I'll try, and I hope the whole thing is in nat or in the specificity of the work of different softphones
Just add

nat=yes
qualify=yes

After that, through the sip show peers command, it will be additionally visible in the Status column
A few recommendations from the article on Habré
And another interesting article about nat and related problems