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.
@kc8apf I'll be surprised if that turns out to be the true reason. I've run some pretty old software on many LTE networks (though not GFi) and never seen that failure mode.
LTE on GF works fine with dotOS 2.5. Put stock 7.1.1 back on, LTE stops working. *Shrug*
Generalistic and moderated instance.