My son noticed our unlocked ZTE Trek 2 HD tablet showed LTE on Google Fi but Firefox claimed it was offline. Best I can gather, AT&T/ZTE stopped shipping updates after Android 7.1. Guessing Fi won't let such an old patch level on the network. Trying to update to LineageOS has been 2 hours of frustration due to bootloader nonsense. Tablets aren't supposed to be disposable.
Generalistic and moderated instance.