Why I got very weak signal with my GSM antenna connected to PCB?
I was going to say "check the power supply", since those things have very spiky current draw and demand proper decoupling and low ripple. They might also be subject to crosstalk through the ground plane.

Do you get better results if you only power up one of the modules?

However, the fact that taking the back off the Faraday cage improves reception greatly suggests that the antennas are doing very little work. If you've stuck the antennas to the metal surface, they're probably too close to it to be able to work properly. Normally you want antennas perpendicular to the ground plane.

What sort of signal strength do you get with the lid on and same antenna, but hanging the antenna in free space > 30cm from the box?

Is the box grounded? To the same ground as the PCB?

Are the track impedances correct? Do you need a cutout in the ground plane around the SMA connector?