Hola Frank
There is an old rule that used to be mentioned when explaining the difference between ser and estar which is based on something being permanent (ser) or non-permanent (estar). This rule is less and less used by teachers because there are lots of exceptions and ends up being confusing.
We explain the different uses of ser and estar in specific lessons so the students learn them and associate that specific use (context) to the right verb.
This sentence you are giving as an example from one of the kwizzes:
Las islas Canarias están en el océano Atlántico.
(The Canary Islands are in the Atlantic Ocean.)
needs estar because this is one specific use of this verb: talking about location, i.e saying where things/people are (placed-positioned/located).
If you click here and scroll down, you'll see lots of Kwiziq lessons on these two very important verbs.
I hope it clarified it.
Saludos
Inma
