I have an "old phone" not on their whitelist that supports LTE. AT&T sent me an "equivalent" (as in the lowest spec android phone they could possibly find) repl

I have an "old phone" not on their whitelist that supports LTE. AT&T sent me an "equivalent" (as in the lowest spec android phone they could possibly find) replacement phone and my old phone stopped working. I registered my sim card as using this new phone, then promptly put the sim into my old phone and my old phone works perfectly even though I am registered with the new phone.