What is the key difference between مِنْ رَبِّهِمْ and مِنْ بَعْدِ?

Sākin Nūn & Tanwīn — Iẓhār, Idghām, Qalb and Ikhfāʾ

• First is idghām without ghunnah; second is qalb
• Both are iẓhār
• First is qalb; second is ikhfāʾ
• Both are idghām with ghunnah