You Do Need power plug adapter while traveling from Congo to São Tomé and Príncipe.

Here is Why You Need An Adapter

The outlet and plug type used in São Tomé and Príncipe are C and F 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

São Tomé and Príncipe

In São Tomé and Príncipe, outlets of type C and F are in use.

Type C

Type F