Thank you.
If I flash the firmware using KIES for the t-Mobile version of Lollipop and then use my AT&T SIM card I should be okay ?
Otherwise I'm not sure I see the advantage in getting an unlocked phone ?
Exactly,you'll be good to go,provided the phone & firmware are both T-Mobile,yes,you'll be good to go.
You may have to contact T-Mobile for a SIM unlock to use your AT&T SIM Card.
Not a big deal,especially if the device is paid for in full.
The SIM Card unlock code from T-Mobile will cost $0.00,contact their customer care for the code & SIM unlock instructions.
If not,there are other methods of getting a SIM unlock code,GOOGLE is your friend here.
Just a couple side notes:
The info how-to's/etc.. I've recommended are based off of general rules of thumb & in this case w/this device (T-Mobile),I recommend staying w/the firmware that came with the phone & only updating w/T-Mobile updates., whether through KIES,or,OTA (if possible w/o service from T-Mobile).
There are always exceptions,as the case w/the NEXUS 6 for the US Market:
You can purchase it unlocked or from any of the US carriers & flash the appropriate firmware for your US carrier.
Again,
This is a rare exception.However,subsequent updates should match the firmware of the carrier you're using,just as the case here w/your phone.
Only reason I mention this is that the AT&T & T-Mobile Note Edge may
possibly be the exact same hardware,only differing in model number only.
However,unless you can verify otherwise,I'd err on the side of caution & assume the hardware & firmware need to match exactly.
Ditto w/future updates.If you have to manually update,ensure the update matches up w/what you have on the phone (T-Mobile>T-Mobile).
So,to summarize:
T-Mobile Hardware > T-Mobile Firmware > T-Mobile updates
The SIM Card actually has absolutely nothing to do w/flashing firmare via KIES,you can leave it in or remove it,doesn't matter.
As I stated in a previous post regarding my AT&T device on T-Mobile:
The phone was never sold/carried by T-Mobile,so,the only way I could update was via OTA from AT&T's servers.
The firmware update wasn't available for a manual update,or,AT& T wasn't willing to share that w/me.The only way I could get the update was to connect to AT&T's servers,which meant getting service w/them (SIMCard).
So,if that particular update was available via a side loaded download,or using update software such as KIES,I would have taken that route & the SIM Card would have had absolutely no consequence in the update whatsoever.