The nomenclature rule that you have missed is given in Section P-46 in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) (explaining that compound acyclic substituents consisting of a principal chain and one or more acyclic substituents can be named in two ways which can be found in Subsection P-29.2) and in Subsection P-22.214.171.124, which clarifies that the preferred names for simple substituent prefixes derived from saturated parent hydrides with a single free valence that is not at the end of the longest chain are formed by using alkanyl substituents according to Method (2) of Subsection P-29.2:
(2) The suffixes ‘yl’, ‘ylidene’, and ‘ylidyne’ are added to the name of the parent hydride with elision of the terminal letter ‘e’, if present, when followed immediately by the letter ‘y’. The locants for the atoms of free valences are as low as is consistent with any established numbering of the parent hydride and, except for mononuclear parent hydrides or the suffix ‘ylidyne’, the locant ‘1’ must be cited. This method is used to generate names of ‘alkanyl-type substituent groups’ that are simple substituent groups with free valences at positions other than ‘1’.
For the substituent that is mentioned in the question, the preferred alkanyl-type prefix according to Method (2) is ‘octan-4-yl’.
Note that, according to current IUPAC nomenclature, the prefix ‘isopropyl’ is only retained for use in general nomenclature but the preferred IUPAC name is ‘propan-2-yl’, which is also formed according to Method (2).
Therefore, the complete name for the compound that is given in the question is 6-(octan-4-yl)-5-(propan-2-yl)dodecane.