I'm with a Samsung Galaxy S5 Mini LDU (SM-G800X). I've flashed an official Firmware of the Exynos Version (800F) got from Sammobile, for remove Retail Mode.
So far so good.
After flashing, something strange happen: on Status Menu the phone shows an IMEI! This is very strange cause all the LDU I've managed in my life (S4, S5 Note3, Note4, and some tablet) no one has an IMEI (obviously).
So, I decided to go through this and try to put a SIM card. Before put a SIM card, I've flshed TWRP for Exynos Version, got a FULL WIPE (System,Dalvik,Cache and Data) and reflashed the official Firwmare.
Now the phone is fully clean like a new one and ready for try SIM card.
I've tried tu put the SIM but no signal is reached. My story is very similar to the @gmedrano84 one. All he said is similar to my story, but the difference is that I have an IMEI so I don't know why this LDU canno't work.
Also on the Boot Screen, the sign "Live Demo Unit" is gone after flashing an official firmware (on my S5 LDU this not happened). The phone on first boot also checked for new OTA upgrades (LDU normally do not do that!).
So I ask you @vndnguyen to help me find the truth on this story. I wanna know if there is a chance to get this phone FULLY Working!
Thank you guys in advance!!
EDIT: To help you I've attached here some screenshot from your PHONE Info app and from the ServiceMode. Hope this can help!