Etm571 https://www.att.com/deviceunlock/
Could this help?
That page appears to be about SIM unlock, in which case it would not help, because installing GrapheneOS requires bootloader unlock, which is not the same thing (as mentioned on the page I provided the link to, which may be instructive if consulted).
Because SIM unlock and bootloader unlock both contain the word "unlock", confusion is possible. But one will not suffice when the other one is needed.