At this point I recommend testing the area around the sim card reader on the motherboard. If you have a microscope, after proper cleaning it should be easy to check for bad pads, lost components, anything out of the ordinary.

Also perform a check with multimeter, using the continuity test mode, to see if the pads actually lead to where they should.

There should be no continuity between any two pads, except for the two ground pads (marked black in the picture). I jotted down a few resistance values for resistances between the pads, in case somebody needs to know them (they are approximates, apparently there is variance to this)- if you need to know these, send me a message.