I solved all those problems above and now the phone has original serial number with a working/tested IMEI and flashed with 4 file firmware 5.1.1 (G920FXXU3COI9_G920FBTU3COJ1_BTU) the only firmware that works with Odin. All other firmware in the support area fails using Octoplus. I tried so many other 5.1.1 firmware files, certs, and EFS files from working S6 models, just incase and all seems to go well.
The phone shows correct serial, IMEI, and all other functions work fine…
BUT… the phone ONLY shows "emergency calls only" "No Network". However, it does nearly always pick up the name of the Sim-card service provider on start up and in the settings. The Signal bars seem to be there but no 4g or H+ or 3G ever shows. When I scan for "mobile networks" manually, it sometimes finds the network operators but always fails to connect to network. IMEI status OK in service mode.
Settings > status shows: network: unknown > Service status: Out of service > mobile network state: disconnected. But the phone pops up with name of network operator of which ever Sim-card in insert. Very strange.
Have been working on it for 3 days straight but unable to figure it out. Any help will be appreciated. 🙂