How do I reactivate old iphone with old sim after transferring to iphone 15 eSIM?

I'm upgrading from an iphone 11 pro to an iphone 15 pro. I'm using the same carrier, same number. The 15 uses an eSIM, so when I activated it, my old physical sim card in the 11 was deactivated.

However, I intend to keep the old iphone 11 (with the old physical sim card still in it) as a backup. If I lose my iphone 15, how do I reactivate the old sim card on my 11?

Is there an easy way to do this or do I have to call AT&T?