Trading forex and CFDs is legal for individuals in Nigeria, and no rule bans bonuses, so the offers you see are mostly the standard global promotions. What is missing is a local licence: the SEC does not currently issue retail forex broker licences in the way the FCA or FSCA do, so the brokers serving Nigeria are almost all regulated offshore by authorities such as the FCA, CySEC, ASIC or an offshore registry.
That makes the licence of the entity you actually sign up with the thing to check, not the bonus headline. A broker may advertise tier-one regulation but place Nigerian clients under a lighter offshore entity. Find out which entity holds your account and what protection, if any, that licence carries before you deposit.
Funding adds a second layer. Because the CBN controls foreign exchange and naira movement, confirm that deposits and withdrawals in your method work cleanly with the broker before you chase an offer. A large bonus is worth little if you cannot get your own funds back out in naira.