I don’t think there’s an Apple service here in Nigeria

If there is no Apple store or Apple authorized service center in Nigeria, then you will have to take it to where there is one.

If Face ID isn't working for you, there is most likely something wrong with your TrueDepth camera. First, look at these two articles for help. If that doesn't fix the issue, then make an appointment at the Genius Bar to have your camera issue diagnosed.

If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

If Face ID isn't working on your iPhone or iPad Pro - Apple Support (AZ)

Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple

Service in Nigeria is provided by the authorized carriers there.

Apple does not provide direct service or support in Nigeria.

That is something you will need to figure out. If you really need it fixed, that is your only option.