I have a friend same situation (weird they are from Nigeria also and had to switch to divorce waiver). She filed one month after my husband which was June 2018. She just had her N-400/I-751 interview 2 weeks ago. Yes filed in 2018.
There's was filing I-751, getting divorced. moving from one state to another, transferring case. But finally after many e-request finally. Still not approved still waiting for I-751 to get approved. It's like it's stuck like yours. Hang in there. Hope you don't have to file WOM.