Iphone 5 No Service

Hello Guys,

I`ve bought secondhand Iphone 5.
On the back cover IMEI and Model nr. does not match. (Wasn`t paying attention when I was buying the phone 🙁 ) (Logic board changed?)

I`ve tried everything, restoring, resetting networks, airplane mode on/off, setting date and time manually, updating to 9.3 beta and other methods with no success.
Sim card is brand new, few days ago I got it, works in another phones.

The guy from what I bought it said – after updating to 9.2, issue started when it was 8.x.x it worked with no problems.

I also have to mention – it has 1 large crack in glass(not screen, screen is working fine)
It looks like the guy tried to change the glass, and that`s why there is a crack, because the UV glue (or maybe other glue that guy used) is not hardened properly.

I thought – easy fix, change the glass and good to go (glass is on the way from ebay) 🙂

But no, No Service issue is driving me nuts 🙁

What I`ve found to get network to work for short period of time – changing LTE to 3g, after restarting phone service is available for about 20 to 40 seconds(but not always), but what is interesting – if i call somebody in that period of time, I can talk more than 5 minutes, but after hanging up, service goes to – "No Service". And also in that period of time I can send messages(when service is available obviously).
And in settings>general>about – Always shows Carrier – LMT 23.0(Latvia) (when service is available or not), but Network – Not available(and of course when its available it shows – LMT).

What I`ve done – reseated antenna connector(with no luck).

Reheat Gsm related Ic`s?
Software issue? (Before I was 100% sure of that, but now I`m not)

Your help will be very appreciated.
Thanks 🙂