iPhone no service, cleam imei, correct date, good antenna, so how?

Hi I am a phone technician and I recently purchased a phone from a friend, iPhone 4s - unlocked.

I was able to use the phone on multiple network without a problem.

I changed the LCD, and attempted to use the phone a few days later but I was greeted with no service..

- Date and time is correct

- Antennas are fine

- Sim cards are fine

- all connections etc are fine.

- Clean IMEI checkmend clear

- Tried airplane mode toggle

- Tried battery

It searches for signal and then says no service, if I go to contacts my number is displayed at the top, I have had it on WIFI and iTunes, I have also updated to latest software, I also did a full reset and set up via iTunes and when it connected it said congratulations your phone is unlocked. No matter which sim card I try I get the same results.

I have tried everything I can think of, does anyone have any other suggestions please?