Please clarify that did you register your account successfully?
And after register your account, you did not get any pop-up notification that your account is blocked, unusable because of your nation is blacklisted.

Or were you not allowed to register your account at MonkeyTilt with an IP address from Nigeria?

It's important question, because your post is like you successfully registered your account and only found that Nigeria is blacklisted (restricted) by checking MonkeyTilt Tos.