All network operators in Nigeria have been directed to update their subscriber records so their National Identification Numbers (NINs) are captured.
Read more at https://ncc.gov.ng/media-centre/news-headlines/928-press-statement-implementation-of-new-sim-registration-rules

Here are some useful tips on linking your NIN to your SIM registration records.

Forgotten your NIN?
You can dial *346# on your phone to retrieve your NIN.
Alternatively, you can visit any NIMC office close to you.

** Note - Your NIN is stored electronically within the chip on your National e-ID card and can only be accessed using an authorised card reader. The number printed on your card is NOT your NIN, it is the Primary Account Number (PAN) used for accessing your card's payment feature once activated. **

Link your NIN to your Airtel Mobile Number
Dial *121*1#
Read more at https://twitter.com/AirtelNigeria/status/1339540960146034689
You can also visit an Airtel service centre close to you.

Link your NIN to your MTN Mobile Number
Visit https://mtnonline.com/nin/
Or you can dial *785#
Read more at https://twitter.com/MTN180/status/1339513283649396738
You can also visit an MTN service centre close to you.

Link your NIN to your Glo Mobile Number
SMS “UPDATENIN NIN FirstName LastName” to 109
Read more at https://twitter.com/GloWorld/status/1339860485571788804
You can also visit a Glo service centre close to you.

Link your NIN to your 9mobile Number
Visit http://www.9mobile.com.ng/nin
Or you can dial *200*8#
You can visit a 9mobile service centre close to you.