[Samsung Galaxy S8+ Questions & Answers] JeffDC:
Just flash the CSC that comes 'stock'/with the rest of the full, stock firmware set, Odin will work. Have the SIM in the phone before you reboot, as for me it seems to help.

I haven't found a difference, for me yet, just flashing the

exact

CSC through Odin or TWRP. As I'm now on a custom ROM, I'm usually flashing via TWRP with a CSC that is supplied by an Aroma setup. Aroma is very handy.

Yes, flash your full stock via Odin. If you have TWRP you will lose it. You can re-flash TWRP from Odin. I typically flash TWRP all by itself, but I don't think that is really necessary. Maybe someone will come along and let us know.

As an aside I always keep a copy of zips/installs of TWRP, re-verity and SU on my SD card.

So after the stock full flash, I reboot into download mode and Odin flash TWRP, and follow that by a direct 3 key reboot into recovery, and TWRP will load. If you are going to root then flash re-verity, then SU/Magisk. You could then flash your CSC, but with the issues you've been having, I'd then boot to system, do the initial setup of the phone, and then boot back to recovery/TWRP, backup the phone.

Then TWRP flash the CSC Aroma package, making sure you have the SIM in the phone and reboot to system. I've been using XAA as my CSC

with my custom ROM

, within that is AT&T, T-Mobile, etc for US phones. I select the main XAA CSC from Aroma. Next I select the unbranded XAA as my more specific/local package, instead of AT&T, for example.

My 955F, stock worked just fine on it's 'stock' CSC (China region). I only had to change the CSC when I flashed a custom ROM, and that ROM did not have the ROM's (model) of XAA, so I had to find a CSC, which had XAA, that 'matched' the ROM I had flashed. I had flashed a Note 8 ROM, CarHDROM,v8 initially. Again, that ROM was largely meant for Europe and Exynos phones, so I had to be pointed to a Note 8 CSC that was compatible with the Note 8 custom ROM.

Yes, Odin is where you would flash stock firmware/stock CSC (not in an Aroma package) for your SM-G955FD. It is my understanding that all non-US phones, eg the G955's, are mechanically the same, so flashing 'any' Exynos ROM, G955FD ROM should work via Odin. You can also flash CSC packages via Odin as well, but there is no Aroma process, to my knowledge/experience. So have the specific and correct CSC to flash via Odin, if you go that route. It sounds like you are aware your phone is Exynos? A non-US phone?

I specifically bought my G955 (8+) as non-US so it could take root smoothly/easily, and work great world wide, including working great here in the US. Sometimes the U.S. does not seem a part of the world.

As I've been messing about with S7's/8s, there have been times that the phone did not grab onto AT&T, for example, and i would then 'reset' the network and reboot, with the SIM in the phone, then do a search for phone networks/services. That usually solved it, but when it did not, and I know I have the right CSC, I would then input/create a new APN, with AT&T, Cricket, Sprint, or what-ever service I needed, then reboot.

The only times I've not gotten the phone/s to grab the networks after that is when I've had the incorrect CSC.

Again, on my stock what-ever country G955F, stock G930S, etc, all Exynos, with the same stock (non-US) CSC that come in the firmware package, the phones have grabbed AT&T/T-Mobile just fine. If it would not, I would have reset the network, rebooted and entered the APN, and then rebooted (with the SIM in the phone). Done a network search after that, if necessary.

Hope this helps.