Hi Mahmoud,
Thanks for sharing your doubts 
What Nico is sharing here is how to enable mesh functionality in 2.4ghz, which means, enabling LibreRouter to transport messages from other nodes over the network using that radio.
What you are interested in is using this interface as an access point (no mesh), so client devices can join the network, and it’s enabled by default.
Regarding your other questions, let me point you to this topic.
It is in spanish but you may find it useful using an automatic translator.
Also, as this is your first post in the forum, it would be awesome if you present yourself, your community and so 
So we get to meet each other.