AdiIRC

Either your dns server is broken or your ip address does not have a reverse dns hostname.

You can check on sites such as https://dnschecker.org/reverse-dns.php

There was an option to add or change it in mirc options 'Local Host'

Is it available in adiirc?

it is not used for anything other than being displayed in that message, why do you need a option to set your external non existing hostname?

Because I get banned from some servers when they read the word "unknown" next to my ip address, as they think I'm using vpn, while I'm not

That's not the case, they can't see that text. It's a text adiirc is showing you after it performs a dns lookup of the external ip address the remote server reports you are connected from.