It appears like you have done everything correctly.
Do you have the original cracked screen? I would try to put the old screen back and test it thoroughly. That way you can see if the problem is related to the replacement screen or the logic board.
The other thing I would check is the antenna flex.
[product|IF268-013]
Take a look at this image and notice it has a small conductive pad.
[guide|29339|stepid=132621]
That is likely meant to ground the selfie camera bracket and, who knows, maybe even act as an antenna. Sometimes, that pad sticks to the bracket more than the flex so it might not have been transferred to the replacement screen. Also, that flex is connected via four connectors underneath the logic board. Maybe one of them came loose.
+
+
EDIT 2018/11/18
+
+
The iPhone 6 has unfortunately developed a “thing” for GSM/LTE connection/searching issues and over time the repair community has found some classic issues. You can read more about it below but it’s entirely possible that the the issue the OP (Rosu) has was coincidental to the screen repair.
+
+
After you have eliminated antennae or network locks as the possible culprit, searching issues on the iPhone 6 are usually related to the baseband IC. The baseband IC is very sensitive, just like the Touch IC's on the 6 Plus; it also suffers from poor quality traces and pads (which could reached the tipping point following a bit of flex during a repair).
+
+
Go to your phone’s dialling keypad and dial *#06#. You should see the IMEI displayed.
+
+
If the IMEI is not displayed, the Baseband IC has to be reballed and two small copper traces (U1/V1) have to be rebuilt and solidified. This is commonly referred to as the “long jumper” solution. You can check out this answer ([post|297144]) (by @teetopp) as it summarizes the issue really well.
It appears like you have done everything correctly.
Do you have the original cracked screen? I would try to put the old screen back and test it thoroughly. That way you can see if the problem is related to the replacement screen or the logic board.
The other thing I would check is the antenna flex.
[product|IF268-013]
Take a look at this image and notice it has a small conductive pad.
[guide|29339|stepid=132621]
That is likely meant to ground the selfie camera bracket and, who knows, maybe even act as an antenna. Sometimes, that pad sticks to the bracket more than the flex so it might not have been transferred to the replacement screen. Also, that flex is connected via four connectors underneath the logic board. Maybe one of them came loose.