Install New Module Link Not There
There are 2 components to the Install New Module UI access callback:


It requires the Administer software updates permission for the user to be enabled. It can be enabled for any user role underneath the People > Roles administration section for the relevant user role.
It requires update_manager_access variable to be set to TRUE. By default, this should be TRUE, but it can be overridden in settings.php or manually set (e.g. drush vset update_manager_access 0). In which case, it needs to be adjusted in settings.php or by updating the variable in the database (drush vset update_manager_access 1).