[LG V30 Questions & Answers] TheDannemand:
Yes and no: The second of my US998 (still on Pie) has not had any changes in settings, and its modem works fine.

The first US998 I bought was essentially the same when I first received it (also on Pie). But back then I compared it to the H932 I'd had before (and which my wife still has). I found that it showed fewer bars than the H932 when connected to the same tower and the phone placed exactly the same place. Turned out measured signal strength was the same, just that T-Mobile is showing an extra bar in their ROM to give the impression that their network is better than it actually is. Pathetic!

Then I used Hidden menu->Field Test->Modem Settings->Band Selection->LTE Band to select only the low frequency AT&T bands (5, 12, 17, 29). This is something I'd always done on my H932 to improve reception in my home and save battery, and it's quite effective.

This particular LTE Band Selection menu was always available on H932. I was aware of the horror stories from US998 users back in the days, who got locked into a fixed LTE band after using the Band selection menu on US998 Nougat and Oreo. But I was glad to see that the TMO style LTE band Selection menu is present on US998 Pie. And it works: My phone then only connected to bands 5, 12, 17, 29 and avoided connecting to band 2, which AT&T prefers, but it takes a lot of power and doesn't penetrate our walls very well.

HOWEVER, I later realized that the Reset function in that menu doesn't work on US998 like it does on H932: You have to always choose a selection of bands. Not a huge deal, but I don't like hidden menu changes which I cannot undo.

I was surprised after rolling back to Oreo (via 10d, Master Reset and BL unlock) that the LTE band selections survived. By wiping modemst1 and modemst2 partitions I was able to make the phone connect to all bands again, but if I use Shortcut Master to load com.lge.hiddenmenu.tmo_only.LTEBandSelection (which IS available on US998 Oreo) it STILL shows the selections I made long ago on Pie (5, 12, 17, 29) even after many Partition DL and Refurbish flashes back and forth between US99810d, US99820b, US99820h, H93310h, and several Master Resets. The phone DOES connect to other bands, so presumably it's just a leftover display of the TMO menu settings. Still, something is clearly never wiped!

At first I thought that might be the reason my Oreo phone has such a sluggish modem, which only reluctantly connects to LTE. But when I am on US99810d or on H931h20 the modem is perfect. It's only when I am on US998 Oreo it becomes slow. So I have concluded that my modem issue is indeed a limitation of the US998 Oreo ROM, not caused by the menu changes I made. But it still irks me. If somebody can tell me exactly which partitions to copy from my other US998 (without losing IMEI, serial number etc) I would be very grateful!

So in short: Yes, I made some hidden menu changes on one of my US998 while it was still on Pie, and it worked fine to limit which LTE bands it connected to. My other US998 on Pie has had no such changes. Both of their modems work well on Pie, it's only the one on Oreo that has problems.

(Sidenote: May I suggest you edit the Title of this thread to fix the typo: rs998 -> US998. On XDA thread owners can do that themselves.)