1) Did you request a combo interview in cover letter when applying for N-400?
2) Did you attach copy of I-751 in your N-400 submission?
I does happen, your case is not extraordinary.
No, you don't have to apply for N-400 again or pay the fees. When your I-751 will get approved, you'll be placed in line for oath.
Legally, if you don't hear anything 120 days from N-400 interview, you can sue USCIS (Writ Of Mandamus) and they'll have to give you a decision.
Hang in there, your journey is almost completed!