There is a wonderful book "Medieval Africa, 1250–1800 by Roland Oliver" it makes a wonderful read on the history of Africa in general with chapters covering each part...

The Portuguese discovered the mouth of the Congo river in 1483. Here is an extract from pg 168:

THE PORTUGUESE AND THE OPENING OF THE ATLANTIC
Such in outline was the situation in western Central Africa when in 1483 a Portuguese caravel captained by the famous navigator Diogo Cão, sailing southwards on a voyage of exploration from Elmina, encountered muddy water several miles offshore and turned eastwards to investigate. Thus Cão reached the estuary of the Congo, called by the local people Nzadi (whence Zaïre), and made contact with the subjects of the Manikongo, Nzinga a Nkuwu. In the course of a later voyage in 1485-6 Cão visited the capital, twenty-three days’ march inland, and then sailed home carrying a party of Kongo emissaries, who on their arrival in Lisbon were baptised into the Christian faith and placed in a monastery for initiation into Western
ways. They were returned to Kongo in 1491 in a fleet of three caravels, carrying Portuguese priests, masons, carpenters and soldiers, a selection of domestic animals including horses and cattle, samples of European cloth and other manufactures, even a printing-press complete with two German printers. The ships anchored at Mpinda in the Congo estuary, and after a brief halt to baptise the provincial governor of Soyo, who was an uncle of the Manikongo, the expedition proceeded to the capital, where Nzinga a Nkuwu and five of his leading chiefs were baptised on 3 May 1491.

Reading further on the ongoing tribal wars,

Of course, despite his Christian piety, Afonso was throughout his reign deeply involved in the capture and sale of slaves to the Portuguese. It was the inevitable price which he had to pay for his European advisers and for the material luxuries with which he rewarded his chief subjects. Without doubt, his slaving wars gave a military impetus to his kingdom and consolidated his authority in the border regions to the east and the south. The condition of success in these adventures was, however, that the trade should remain a royal monopoly. Hence his growing disquiet with the Portuguese sea-captains, who tried to trade directly with his subjects and even with his enemies.

With the weakening of the Kongo kingdom following
the death of Afonso, the raided became the raiders, joining forces with the Mbundu, not only for the defence of Ndongo but in many subsequent invasions of Kongo territory. These began during the reign of Afonso’s grandson, Diogo (1545-61). His successor, Bernardo (1561-7), was killed while fighting the Yaka on the eastern frontier. The next king, Henrique (1567-8), died in a war against the Teke. His successor, Alvaro (1567-8), had barely acceded when the whole eastern side of the Kongo kingdom was laid waste by a great invasion of Yaka war-bands, which swept through the provinces of Mbata and Mpemba, capturing and sacking São Salvador itself in 1569.
Alvaro fled with his courtiers to an island in the lower Congo and sent desperate appeals for help to his Portuguese allies.
Meantime the Yaka made profitable contact with the São Tomé traders at the ports of the Congo estuary, to whom they sold thousands of their Kongo captives, including members of the royal family and other notables.

HTH