My issues is quite complicated. Please be patient and read on.
Less than a week ago, I bought an LG G3 D855. It wasn't brand new and definitely out of warranty.

It was on the 4.4.2 Os version but I couldn't update it because I couldn't locate the UPDATE CENTER and after some research online, I found out it was because it was rooted. I couldn't find a Supersu application anywhere on the phone anyways but Root checker confirmed that it was rooted.
I downloaded the stock kitkat firmware in a .tot format and I tried flashing using LG Flash Tool for TOT but it failed because "Device version is different"
I checked the Imei online and it stated that the device is a Sprint LS990 but the details embedded in the phone all say it is D855

I checked online and found a couple of posts about this error and one user said he was able to fix it by flashing the actual Sprint .tot with the lg flash tool. I tried that but it didn't work. It still said "Device version is different. Check BIN or dll"

I checked online and found results pointing to a generic .dll file for G3s and I downloaded it but when I used it on the Flash tool, it resulted to an error "CrossDL blah blah"

So I found a Marshmallow kdz file. LGUP won't see my device model so I was able to flash it after editting the software in my build.prop to D85510n.

***I have always changed the port to COM41 in all the steps I used

The LG UP detected my model and it flashed to 100% but got stuck on a bootloop by loading the lg screen, freezing and restarting by it self. It never gets to the homescreen.
I downloaded lollipop kdz reflashed using LGUP in a form of downgrade and it got to 100% and got stuck at the lg screen while trying to reboot.

I redownloaded the stock kitkat rom and used the Lg flash tool but it failed. It said 'PID null IMEI Null"

I used LGUP to reflashed. It came to my understanding that the LGUP can flash both kdz and tot. So it flashed to 100% but got stuck at the lg screen and I have tried all I can but it is stuck. It can go into download tho. I don't know what to do about the IMEI. I don't know if the last flash retrieved it. I don't kniw what to do.

It no longer shows as LGE AndroidNet USB Serial

Please can anyone help me?
I just need my phone to work again. I was stubborn and now I regret it. I should have just stuck to kitkat. I'm so desperate to get my phone working that I wouldn't mind Android Froyo Os if it was ever possible.

PLEASE HELP!!!