locked firmware needs to be signed by hardware manifacturere afaik so GrapheneOS cant touch this, and also cant update them (which is why EOL devices are EOL for good).
Please stop spamming here and try the official Google testing utility.
Then if that does not detect anything, try emptying the device until the battery is dead, keep it off for a day, charge it again. Just a random idea.
If that does not help, make sure you reset the OS to stock and make sure you dont disable things on startup.
NLP is controlled by Google servers. They can see your device ID, since you switched to GOS for a while, it is just like you had your phone off for a while.
Maybe it does not work if the device is bootloader unlocked or random other issues, in the OS.
Make sure you really have reset it to a stock state. Working SIM Card, locked bootloader, etc.