DMZ is a networking preference and it does come with security limitations. While we do not have a tutorial ourselves, we recommend reaching out to your internet service provider if you need any assistance in setting your console in DMZ if you are not entirely familiar with the process.
I'm afraid we do not have any updates to share right now on a fix for this in the near future, but this specific "Party Not Ready" error is indeed still being tracked and investigated by the Apex Legends team.
Setting console IP as DMZ was mentioned earlier, and unfortunately it does not work for my case (also reported by others here)
and you are right, users really should not set it this way it’s bad practice and may come with its own bag of problems.
Setting your console in a DMZ will bypass your routers firewall and leave it open to attack. Not something that should be used if it can be avoided.
Alternatively respawn and EA should sort themselves out and actually fix the issue.
Only way they will take notice to these problems is if players stop playing the game.