[LG V30 Guides, News, & Discussion] TheDannemand:
I understand your problem. But as pointed out, you can NOT cross flash H932 with anything else. Period!

As I see it, you have the following options:

1) Change back to a MVNO that uses ATT if TMO has poor coverage in your area. I use Red Pocket, which has a number of plans on all four carriers (VZ, ATT, TMO, Sprint). ATT based plans are generally the least expensive. My plan has 100mins/100txt/500MB per month at $60 for 12 months ($5/month no tax). My wife has their regular monthly unlimited/3GB plan at $19/month. Best deals are on the Red Pocket eBay store, such as this unlimited/7GB for $20/month if you're willing to prepay for 12 months. Red Pocket provides VoLTE and VoWiFi on all plans (some limitations on Sprint). US998 supports VoLTE alright, but only VoWiFi on TMO.

2) Convert the phone to Canadian H933, which has the most LTE bands of all North American models. It has all the same bands as H932 except B40 and B41 (high frequency) and B71 (low frequency). You have to do a proper Frankenstein: Partition DL via Nougat US998, then Nougat H933, then Oreo or Pie H933. Personally I couldn't get the radio to work when I was briefly on H933, but I believe others have made it. I don't know if/how H933 works with VoLTE and VoWiFi on various carriers.

3) Use the Band Selection Hidden menu to choose which LTE bands to use and NOT use. TMO's main band is B4 (1700MHz) which is fast, but doesn't penetrate walls very well. By forcing the phone to connect to low frequency bands (such as B12 and B29) you will get better signal indoors, at the cost of lower data speeds. You will save some battery too compared to the high frequency bands.

On Pie, you can access the Band Selection menu through Hidden menu: *#546368#*998# -> Field Test -> Modem settings -> Band Selection -> LTE Band -> (Select bands you want to use) -> Save (don't forget to click Save).

On Oreo do NOT use the Hidden Menu or you'll get a nasty single-band only band selection, which has caused much pain in the past. Instead use Shortcut Master, search for LTEBand and choose com.lge.USCHiddenMenu.iot_hidden_menu.LTEBandSelection.

Do note that the Reset function in that menu doesn't work on US998 (it works on H932, maybe other models too). And as far as I can tell, neither Master Reset nor Factory Reset will revert these selections to default: EVEN if the the menu shows up blank after reset, the selections are still present in the modem. On a rooted phone you can wipe modemst1 and modemst2 partitions. Possibly a LGUP Refurbish flash will clear them. Otherwise you'll have to keep selecting the bands you want to use.

Much should change next year, when TMO starts putting their newly acquired Sprint spectrum to use. It's all about low frequency bands, and TMO has had too few of them in the past, mainly B12 (overlapping with B17 on other carriers) and B29. Until the Sprint merger the much ballyhooed 600MHz B71 was their Plan A, but that band is only slowly being released by its former broadcast users. Now they also have Sprint's 800Mhz Band 26, but older GSM phones don't support it. I don't know what the plans are for repurposing Sprint's CDMA bands -- which would cut off current Sprint phones.

Anyways, you have some options. I would just choose a carrier that works better in my area :cowboy: