[AT&T LG Optimus G] br0adband:
Seems to be an issue with this device, perhaps it's doing something to the SIM cards (who the hell knows) because mine did/does the exact same thing, and here's my particular example:

When I first got this LGOG last Sunday (8 days ago) I put my T-Mobile microSIM in it (one of the TM9190 series SIM cards) and got LTE off the bat without any issues. I kept the stock ROM on here for the first day or so and never had any issues and then I got into doing the typical root/unlock bootloader/install recovery/backup stock ROM/flash custom ROM/etc routine and even after that I was still able to get LTE working almost immediately.

A few days ago I noticed that the LTE wasn't being picked up, and it hasn't since. I've flashed several radios for testing (.33, .48, .33-.84 hybrid, etc) and the ones that are LTE ready still didn't get LTE so I just flashed the whole thing back to complete JB stock (JZO54K, the last available LG build) and even that didn't help - I've yet to see LTE show up in any respect at all with that SIM card.

Now the interesting part:

I have another microSIM that had prepaid service a few months back, bought it at the same time as the one I use for my primary service - they were on sale from T-Mobile online for $.99 each so I got two and kept the other one around just to have it.

In spite of all the flashing I've done in the past few days, with like 5 ROMs tested, a kernel or two, and maybe 7 flashes back to pure stock using LGNPST and the LGE970AT-01-V20j-310-410-MAR-08-2013 ROM (the stock one), after all that flashing torture I put this LGOG through, as soon as I put that other SIM card in it - which is not active at this time - and powered up the phone I got an LTE signal right off the bat without a problem. Had already made the APN changes to fast.t-mobile.com so they just kicked right in and wham, 4G LTE right there on the status bar.

And yes, the SIM cards are the same "series" (?!?!) with TM9190 imprinted on both of them - they also have the same code on the front side (620048) and their serial numbers are just a few digits off in sequence - like I said, I bought both of them with the same order so, it stands to reason they'd be from the same batch.

Loaded a few web pages on the T-Mobile prepaid billing site (it's what comes up when you insert a prepaid already activated but not presently in use SIM, very limited but it does work), checked the system stats, everything showed me I was connected to T-Mobile with their LTE service, this phone, and that other SIM card including the status bar icon being the traditional 4G LTE one (big 4, little G, little LTE under it).

Then I rebooted the phone and that was that, the LTE capability just up and vanished like a fart in the wind, and I've done most everything that I could do - including reading a few hundred posts in two dozen threads - to get it functional again and it just doesn't work anymore. :(

I suppose it's not a big deal to me since T-Mobile's HSPA+ is fine with me, I regularly pull in 10-15Mbps on a speed test in various locations (was out earlier today testing stuff in the eastern parts of Vegas), but I am somewhat pissed that I am unable to get that LTE functionality working again.

Tried:

- a complete stock flash, several times, of the AT&T JB ROM, no dice

- all the currently available LTE-capable radios, no dice

- an AT&T SIM but it's not provisioned so it doesn't work at all (T-Mobile doesn't do that, hrmmm... their SIM cards work months later even after the number has been assigned to another customer, go figure)

- all the hidden menu options (3854#*970#, WCDMA automatic, tried forcing LTE only, several other things), no dice

- tried manually setting LTE only with *#*#4636#*#* using custom ROMs, no dice

- adding the LTE enabling lines to build.prop, no dice

Nothing works, so I'm only left to conclude that either a) T-Mobile is doing something funky (highly unlikely), b) something is borked with the phone (potentially true), or maybe c) the phone is doing something to the SIM cards after a reboot or any change in the network mode from LTE to anything else (leaning towards this possibility more).

I can't explain it, I can't figure it out, and I can't get LTE working anymore but as I said, the HSPA+ is enough for me as here at home the LTE signal is somewhat weak (1-2 bars at best whereas HSPA+ pulls in 5 bars and a -61dB signal almost constantly).

I wish I could figure it out because I'd like to do more testing with LTE enabled but apparently once LTE gets disabled for whatever reason after having it actually working, not once but twice now it just dies and never becomes possible again.

Stranger things I suppose... :D

If anyone does ever figure this fiasco out, post whatever solution you came up with or discovered so the rest of us can give it a shot as well 'cause I'd love to be able to enable LTE if I so chose. This is pretty frustrating in the long run - if it never ever worked at all that would be one thing, but having it work then suddenly just disappear never to work the same again, that's just pooched. :p