2nd hand (t mobile uk ) lte won't find network ( 3 uk ) worked f 2 weeks now nothing ? 3 uk !!

Hi All.
I'd just like to check to see if anyone has a solution before sending back to Samsung !! I bought the lte version privately because i wanted to take advantage of 3 uk not charging extra for 4g when it comes on line :-) It was on T mobile (uk) but i unlocked it via the service menu (persho off) & it worked fine for about 2 weeks then Sunday morning when I opened it up it connected to 3g+ as normal then 30 secs later it showed no network and emergency calls only - it did this all day everytime I took it out of standby !! I went into network settings - search for networks & it doesn't even show t mobile let alone 3 - it does show o2 & vodafone uk ?? I changed from auto to 3g connections but no change ? I went into 3 shop to check the sim was working properly - was fine ! Went into t mobile shop and they checked their sim and it showed on networks available but wouldn't connect - didn't know what problem was and said even though I asked the guy I bought it from to send it through them they wouldn't repair because it had been unlocked !! I've done a factory reset so is on standard settings and tried the unlocking thing again but to no avail !!
Does anyone have any ideas I can try to resolve this issue before sending back to samsung on monday because I'm not sure if they will fix it under warranty because I'm not the original owner even though it's only 4 months old :-(
Thanks !!

Sorry - first time post - in wrong section & don't know how to move it :-(