LG G FLEX 2 LS996 U+LTE on Reliance CDMA india - Android Q&A, Help & Troubleshooting

Hi XDA community,
Few months back i had purchased LG G Flex 2 for my dad who is a reliance CDMA subscriber and was looking for a decent phone which is hard to fimd in india.
i had bought a phone from a guy who specializes in imported cdma phones.
Everything was working fine till end of April, 2016. We had received a message from rcom saying they upgrading all their cdma customers to 4G. So we need to get a new sim from reliance store.
So i was worried if my dad's phone would work on gsm too or not?
So i tried to put the GSM sim in the phone and check if it works as the phone supports both gsm and cdma.
so this is what i did,
1. in mobile network selected gsm/lte and removed the sim card which in turn automatically rebooted the phone.
2. while the phone rebooting i swapped my cdma sim with vodafone gsm sim.
3. the phone received the signal and i was able to make a call. i was delighted.
4. now i selected the cdma/evdo option in mobile network and followed the above process and re swapped with reliance CDMA sim. But to my surprise the phone shows "Service Disabled" , whereas same CDMA sim worked fine with my CDMA only hanset.
Also, i was not able to check the network option as the phone disabled the Mobile Network option.
5. Now i tried with another reliance CDMA SIM and swapped with the actual reliance CDMA SIM. The phone showed the signal and I could make a call.
6. Again i reswapped the new Reliance CDMA sim with my vodafone gsm sim. To my surprise the phone was showing "Service Disabled", whereas it worked fine on gsm only phone. So i also tried with the reliance CDMA sim that had worked and the phone said "Service Disabled". In the whole process of swapping reswapping and juggling between so many sims, I forgot what option was last checked in mobile network.
7. this strange behaviour seems as if the phone is accepting the New SIM and rejecting it once swapped reswapped with something else.
NOTE :
1. Phone when purchased was already rooted having the following app already installed 1) root browser, 2) supersu 3) shortcut master(lite).
2. Phone is on android Lollipop and software info says LS996ZV7
3. The phone backcover says LG U+LTE x8, and i m not sure if its a sprint variant.
I am trying to search web since last 3 days on the issue but nothing is helping.
Its my dads business phone and he yelling at me for screwing it. He is using an alternate phone for time being but I wanna resolve the problem at earliest.