Yes, there undoubtedly is a paradox here, and it's a paradox that's expressed by all works of art to some extent. Works of art are material objects, existing as we all do in time and space. Yet they are much more than that; they express the spirit of the artist who created them. The work of art allows this spirit to live on long after the artist has departed. In that sense the urn, like all works of art, is both timeless and located in time; transcendent and imminent. Here as elsewhere in his work, Keats explores the tension between these groups of binary opposites.
The urn is a great object of beauty; not only that, it depicts a joyful, riotous scene of gods, mortals, and comely maidens. There is passion, there is music, there is "wild ecstasy." Yet at the same time, the action displayed by the urn is frozen in time. The figures depicted on the urn appear to live, engaged in festive revelry, but the urn itself is silent, a cold artifact from a distant past. In that sense, it does indeed present us with a "cold pastoral" as Keats describes it. But paradoxically the urn is still able to speak to us thousands of years after it was made. And the message that it gives us is as simple as it is profound:
Beauty is truth, truth beauty,—that is all Ye know on earth, and all ye need to know.