When still nun or tanwin is followed by lam (ل) or ra (ر) in the next word, the rule is idgham without ghunnah. The separate n and its additional nasal resonance are dropped into the strengthened following consonant. What becomes prominent is lam or ra, not a nasalized nun.
«مِنْ لَدُنْهُ»
From His presence. Assimilation into lam without ghunnah.
«مِنْ رَبِّهِمْ»
From their Lord. Assimilation into ra without ghunnah.
Compare: «مِنْ مَالٍ» uses nasal assimilation because mim follows, while «مِنْ لَدُنْهُ» uses assimilation without ghunnah because lam follows. In both the separate nun disappears, but the nasal treatment differs.