I have the LG V35 (Amazon version), using it on AT&T.
Symptoms - the phone worked fine with my AT&T SIM before I took it out of the country, worked fine out of the country with a Three SIM, but now I am back I cannot get cellular data on it with the AT&T SIM in it (calls & texts work fine, as does Wi-Fi data). After trying a bunch of things, I broke down and did a factory reset.
That did
not fix the problem

.
I have several other phones on AT&T, and they are all working OK with respect to data connectivity. I put my SIM in a V20 and data works (implying there is not a problem with the line provisioning or the SIM). I put that other line's SIM in this phone and it failed (implying there is a problem with the phone).
I checked the APN after the factory reset and it is enabled and correct (compared to my other Android phones). All other settings are normal (mobile data enabled, mobile data usage not limited, 2G not disabled,
did I miss any?). I tried Network settings reset - didn't help.
However, Settings > General > About Phone > Network has some odd items in it (in bold):
Network: AT&T
Signal strength: -118 dBm 22 asu
Mobile network type: LTE
Service state: In service
Roaming state: Not roaming
Mobile network state:
Disconnected
IP address: Unavailable
Is there another setting (like in the radio or something) that survives a factory reset? Is there a chance that either Three mucked something up, or AT&T when I returned? I have used Three SIMs in other phones (Nexus 4, Nexus 5) in the past, and had no trouble getting those phones back on AT&T when I returned.
Not that this should be necessary, but I made sure that the phone was never powered up overseas with the AT&T SIM in it, so as far as AT&T is concerned the phone just disappeared for a couple of weeks, and then re-appeared.
Thanks.
PS: this is sort-of related to my post in
LG V30 (AT&T) stopped locating itself regarding my wife's V30, however the symptoms are different, and the circumstances leading up to this are different. And hey, at least location is working on my phone (unlike my wife's V30 in the other thread)
PPS - should I be concerned that my working phone is on Band 12 (-106 dBm) while the "dead" phone is on Band 2 (-119 dBm)? (this info is from the LTE Discovery app).