You Do Need power plug adapter while traveling from Congo to Guinea-Bissau.

Here is Why You Need An Adapter

The outlet and plug type used in Guinea-Bissau is C and in Congo are C, D and E. This is how they look like.

Congo

In Congo, outlets of type C, D and E are in use.

Type C

Type D

Type E

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C