And there is a realm outside the worlds which exists and does not exist at the same time because existence comes up with the time and the world. “Outside” of the worlds and time there are no time, no worlds and “existence” is not applicable there.

It is not eternal as being eternal means to exist forever and therefore to be under the power of time. The realm is not not-eternal as it is the only origin and the only reality, other realities being a product of the realm.

It is not limited as having limits mean a space exist, a length, a duration, a movement a time, a change, a measure.

Nothing from our worlds and other worlds is there:

Where water, earth,
fire, & wind
have no footing:
There the stars don’t shine,
the sun isn’t visible.
There the moon doesn’t appear.
There darkness is not found.
And when a sage,
a brahman through sagacity,
has realized [this] for himself,
then from form & formless,
from bliss & pain,
he is freed.

AN 4.50