Try nobel romAfter more than 20 flashes, I finally decided to give up on this phone.
1. I've tried to flash Project Elixir into my Note9 many times, no matter what I tried (I've strictly followed the instructions in the guide), the results are all failure. TWRP prompts
"Updater process ended with signal: 4"
"Error installing zip file '/sdcard/PixelExperience_Plus_crownlte-11.0-20211015-0821-UNOFFICIAL.zip'"
2. I've tried to flash the stock rom first and then back up the vendor, then flashed the Pixel Experience Plus, and then restored the previously backed up vendor as you said before, but there is still no mobile signal after flashing.
I've tried a lot of other things besides the two above, and the results have been frustratingly frustrating, either with no effect or with all kinds of failures.
As I said at the beginning, I've been tossing this Samsung Note9 for 2 or 3 days, and it's time to give up. Currently I flashed it back to the latest stock rom via Odin, and the good news is that everything is "normal" at the moment, the bad news is that this is Android 10 and that's not what I want. Anyway, at least it's a functioning phone, that's enough.
Finally, thank you very much, you've helped me a lot in these days, thank you very much.