I just did this with an ATT LG g4 which I bought to use on on my existing cricket account. It was not getting the Marshmallow update and this is what I did. I had a older un-activated sim from an ATT gophone and I activated it on-line. I had to register but you do not need to give any credit card details - in fact it looks like you could give any info you want - perhaps just a real email address. The completion of registration gives the sim a phone number but it does not work as you have not put any $ on the account and it will un-register if no $ have been added in 28 days but this is no problem.
However, after installing the sim overnight it did NOT upgrade to MMallow. So, I pulled out the gophone sim, put back my cricket sim and lo-and-behold, as soon as it booted up, it said an upgrade was available and it upgraded to MM. Whether it was the re-boot or putting the cricket sim in, I do not know but I'm on MM now. Note you can buy inactive ATT gophone sims for about $5 online.
bluehills5, I’m a long-time lurker, first-time poster to this website. I just created an account to say THANKS a million. I read your comment last night after struggling to get my AT&T LG G3 (on Straight Talk) to update past Kit Kat 4.4.2. After purchasing my phone from Best Buy in 2014 outright, I unlocked it to use on STALK. After a few weeks I rooted my device thinking I would flash roms as I have in the past with my HTC DNA, but I kept reading some of the G3 ROMS had small issues that kept me away. So as an HTC fan I winded up putting the Blinkfeed launcher on my G3. I was happy and content until last week when out of the blue I got a notification about a software update. I had read that marshmallow had begun to roll out for the AT&T LG G3 so I let the excitement get the best of me. Obviously proceeding with the OTA update on a rooted phone was a mistake and it had me in a boot loop. Thanks to Max Lee's instructional video I was able to flash back to stock. Once back at stock, I thought I would get the update notifications for sure but they never showed up. Once I read your email, I just went by Wally World bought the ATT Go Phone BYOP kit for $9.88 and followed your activation directions. Once the sim was activated and I got my GO Phone Number assigned to the sim I went ahead inserted the sim card and rebooted. Then I turned Data on (which strangely worked because I never really gave my credit card info during activation like you said) and I checked for the OTA update. BAM! There it was. So I turned WIFI on to download the OTA update. My software is updating as I speak. First update just completed for Lollipop 5.0.1, now downloading a second update 205.43MB, so far so good. Just wanted to chime in and let others know your method worked for me.
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