[Samsung Galaxy S9 & S9+ Cross Device Development] AlienSexGod:
OEM off in Download mode means OEM unlock switch is ON!

Maybe I am too late to assist you? If so I apologise but maybe someone else may benefit from the infomation?

I currently have another G965F here as a replacement to my first since it had intermittent call drop out and caller voice silent issues (so does the replacement manufactured 5 weeks later but that's another issue) so I may 100% confirm:

that when OEM off is displayed in Download mode means OEM unlock switch is ON!

when OEM ON is displayed in Download mode means OEM unlock switch is ON!

After my first 7 day OEM jail I hit the OEM unlock switch as soon as it appears which forced a reset of my phone and a complete wipe. The OEM unlock stayed on BUT it made the phone incapable of accepting TWRP in Odin. What I should have done is NOT agree to restart and wipe the phone I should have immediately forced Download mode and installed TWRP. What I did, however, was wait another 7 days till OEM unlock switch appeared. When it appeared it displayed the switch had remembered that it was ON. For whatever reason after that extra 7 day OEM jail wait period after booting into DL mode I was able to install TWRP and all the extra patches you MUST install before you do a normal System boot or you will be back in OEM 7 day jail.

Turns out it is normal for Samsung to force a reset when you toggle OEM Unlock option.

BUT DO NOT BE TRICKED. You SHOULD NOT complete the whole reset process. (reset, let it reboot, BOOM RMM Prenormal is back and you are in 7-day jail again)

Every reboot is a chance to flash TWRP or boot into TWRP. DO NOT ALLOW your device to boot into system before you boot into TWRP and flash the patch.

(Use "VOL-DOWN" + "POWER" + "BIXBY" to boot into download mode, "VOL-UP" + "POWER" + "BIXBY" to boot into TWRP, "VOL-DOWN" + "POWER" to force a reboot)

Then follow the TWRP guide of additional patches and multiple TWRP boots.

I ended up with a rooted S9+ but then I installed FLash kernel. After that I could not install a custom rom (continual boot loops) nothing worked most guys in this situation went back to OEM stock via Odin and went back to OEM 7 day jail. For me I was lucky to try [ALEXNDR] * U1ARCC * DevBase v5.5 * rom. This rom has extra cleanup routine in the install which successfully allowed a clean boot after rom install with no boot loop. Thanks, Alex. what a legend!

Unfortunately, I discovered that the phone dropouts and voice going intermittently silent for 10 secs were not accidental there was a problem with the phone. Since my replacement still has the same issues I assume that it may be the modem. Yes, I am running the latest ARCC update which is meant to fix call issues. It doesn't. Alternatively, there is a hardware flaw in all S9+. I am on a reception border where 4G is very weak but 3.5G is strong and I think this area and the phone being unsure which connection to use highlights the problem!

Yes and in your case it appears that your successful TWRP flash but with a normal sys boot without RMM patch etc turned the switch back off according to the info I just detailed in the question above. For me the switch stayed on, however, I was unable to install TWRP until I waited out another 7 days. Once you get TWRP going with all the additional patches and root I would recommend ALEXNDR's rom because of his additional clean up routines for now. Good luck and may you phone not have dropouts in handover areas!