the springs and paper shim and balls help force the cracked loose solder balls down, A heat gun or reflow oven melts the solder and makes it permanent.
Things like badly fit batteries or being flexed in your pocket flex the circuit board and break loose the weak-process solder balling especially on that modem chip.
I assume it was a chip fab or something form when 0% lead soldering was still working out the problems with that process. If you were brave you could hot air it off solder paste both surfaces clean really well with solder wick and see if you can do a new leaded reballing.