Hi,
I have seen your home page:
- with a PC, there is a login link at the top right corner (not smartpocket template);
- with a smartphone, the login link is to find in the top right menubar (smartpocket template).
Menubar > Uprawnienia > Logowanie

You want to display a more visible login link, but I don't see any settings to make it possible.

Idea #1
If you feel confortable with writing some code, you can edit template code in order to display a login link right after link to close menubar X:
In file (piwigo)/themes/smartpocket/template/menubar.tpl
after line 2

<li data-icon="delete"><a href="#menubar" data-rel="close">{'Close'|@translate}</a></li>

before line 3

</ul>

insert line

{if isset($U_LOGIN)}<li><a href="{$U_LOGIN}">{'Login'|@translate}</a></li>{/if}

PS : I've got inspired by file (piwigo)/themes/smartpocket/template/menubar_identification.tpl


Idea #2
In order to redirect to identification.php page any guest user, it is possible to craft a minimal extension with this code inside main.inc.php:

if (is_a_guest())
{
    redirect(get_root_url().'identification.php');
}