[LG V30 Guides, News, & Discussion] ChazzMatt:
You actually lost fastboot mode when you flashed back to H931 firmware (this ROM). Of the North American variants, only US998 has fastboot flash commands. This is why you have to "Frankenstein" H931, H933, AS998, LS998, VS996 to US998 in order to unlock bootloader, and install TWRP.

You then RE-GAINED fastboot then when you flashed back to US998 via LGUP, but you did it the wrong way. Since you used LGUP and wiped out TWRP custom recovery, the only safe way with LGUP to get back to US998 is to Frankenstein first to US998 Nougat, Master Reset, then update back to US998 Oreo. (Yes, I'm aware there's one North American variant which can go straight from their carrier Oreo KDZ to US998 Oreo KDZ, but it's not the AT&T H931 and it's best not to encourage that.)

When you originally flashed the H931 ROM you were converting back to AT&T H931 firmware -- but this time through TWRP, instead of LGUP. For some reason that didn't work for you. You're SURE you weren't using "newest twrp" BETA EXPERIMENTAL 3.3? It's not stable and doesn't work with Oreo stock-based ROMs at all. That's why I'm confused you described the TWRP you used as "newest", if you instead used 3.2.37 stable -- because that is NOT the newest, but is the best. Others have had documented issues trying to use the beta experimental 3.3 TWRP (built on Pie sources).

Anyway, when using LGUP, you need to follow the Frankenstein procedure to safely get back to US998. After originally flashing the H931 ROM then you tried to use LGUP to flash the US998 20H KDZ. Instead you should have gone back to US998 Nougat, Master Reset, then flash US998 20H.

Stock recovery, probably. You flashed US998 20B KDZ via LGUP. You had full stock. You then booted to temp TWRP via your computer and installed this ROM. But this ROM depends on you already having TWRP installed. So, you probably still have stock recovery from US998?

Your bootloader is still unlocked and you are running a debloated stock-based ROM, made from H931 firmware.

When you were in the temp TWRP, did you flash Magisk? I'm curious if you're rooted or not...

Your bootloader is still unlocked and you are running a debloated stock-based ROM. You can't go back to "factory" as AT&T does not publicly release KDZ. This ROM was hand-built by the dev to be debloated stock firmware so AT&T customers can access AT&T options like Wi-Fi calling.

Trying to install an OTA update will probably mess it up, yeah.

Uhmmm.... you're not going to be able to restore the phone back to stock factory H931. Because AT&T doesn't release KDZ images. If they did, you would merely flash AT&T Oreo KDZ via LGUP.

All you are going to be able to do is flash back to stock US998 via LGUP, use fastboot command to re-lock bootloader. At that point you should no longer have the unlocked bootloader warning. Then, sell the phone on eBay for like $185 - $200. That's current going rate for great condition US998.

Many months ago some people were able to use LGUP to flash Verizon VS996 "resources" and resources_bak partitions in LGUP via Partition DL mode, choosing ONLY those partitions to make the bootloader unlock warning disappear. Because the bootloader unlock warning lives in that partition and VS996 doesn't have it. (Verizon never thought the phone would get bootloader unlocked.) It worked for several people, but then someone messed up their phone. Turns out you had to be on comparable firmware. Like you couldn't be on US998 20E and use VS996 20C or you would brick the phone. So, people stopped doing that because as more updates were released, it's hard to keep track of which one could be flashed.