[G4 Q&A, Help & Troubleshooting] berkantkz:
So, recently, I've been struggling with a strange issue which I think that it's a hardware issue. Let me explain you all the things I've done so far.

I was on LOS 14.1 and wanted to go back to stock rom. I had used Nougat for long time and never used stock MM on my G4. I just wanted to give MM a try. Maybe it could give me better battery life and performance. I don't know.

Since I have an UsU'd H815, I properly flashed stock mm boot, mm system, mm modem and then the UsU H815 baseband. However, when the device boots, I got "com.android.phone stopped" warning all the time. Then I went into recovery mode, flashed UsU mm boot and stock MM modem. Now, I got cellular working, set up everything but then I realized that camera didn't work this time. So, basically,

Stock boot + stock modem + UsU baseband = no cellular - camera works

UsU boot + stock modem + stock baseband = cellular works - no camera

Stock boot + stock modem + stock baseband = must crash with blue screen. But now, it just boots and everything works except cellular

Thinking that since I had on LS991 bootloader and maybe it is necessary for both baseband and the bootloader to be the same model/version, I flashed LS991 baseband. During boot animation, the phone got an instant reboot. I don't remember if it booted to either that blue modem crash screen or android system. Most probably it was Android system. Anyway, cellular didn't work anymore.

I've flashed whole backup (except aboot, laf, raw_resources) that I had created with SALT before UsU'ing my device. Still no luck. I always get null imei from *#06# and unknown baseband. When I launch hidden menu (*#546368*#815#), I also get force close on SVC Test > Version info. The information there about baseband, hw version, and NT code say that 'should be implemented'. So far, I think that it's a hardware issue.

Well, but with

cat /data/loggers/modem**a file including modem in its name**

, I see my IMEI and factory version. Somehow, some things recongize my imei number. I also see my imei on imei screen (power off the phone and hold down vol -, vol +, and the power button at the same time until it gets into imei screen).

I tried erasing modemst1, modemst2, and fcg which a reboot followed that proccess as it's stated and confirmed by some others to work here (ofc I restored my backup, still no luck).

Everything made me think that it was a software issue until I see this thread. The screenshot that the op shared on his 2nd post is same with mine. It says null for everything. And his 8th reply and the 3rd option I listed above (Stock boot + stock modem + stock baseband) makes me think that this is totally a hardware issue. Why the 3rd option makes me think that this is a hardware issue is that some/a part is damaged on the main board, and the device/chip/whatever cannot connect to it. Thus, cellular service cannot start, and since it cannot start, the things that must be going wrong with modem just go well.

I feel like I'm ready to try anything (which won't make it completely dead) to get cellular work on my G4, including flashing LS991's ARB 2 TOT file. I'm open to new thoughts and ideas.

Edit: So, according to my researches, I have found out that RX Module is about cellular (@madiweb also stated that on his 8th post on his thread I linked above). I've checked the service manual of H815. It is told there that if there is no gsm/wcdma/lte signal, RX Module (EAT62994301) has to be replaced. I'll ask to LG Customer Care and surely will add info here for the ones who need it.