When still nun or tanwin is followed by ba (ب), the rule is iqlab, meaning conversion. The n-like sound is converted toward mim with ghunnah. Do not pronounce a fully clear nun as in throat clarity, and do not change the ba itself into nun.
«مِنْ بَعْدِ»
After. Still nun before ba requires iqlab.
«سَمِيعٌ بَصِيرٌ»
All-hearing, all-seeing. Tanwin before ba takes the same rule.
Some Qur'anic prints mark iqlab with a small mim. This indicates the pronunciation rule; it does not mean arbitrarily adding a new independent written letter. The phonetic conversion follows the received rule.