A volt meter would be a great thing to have on this one. We can tell if a device is receiving power when we use on connected to a charging cable. To me it sounds like it has to be something with either the charge port itself or there may be a faulty chip on the logic board. Apple uses a charge IC chip that can sometimes go bad. Not sure if there is something similar here. One simple troubleshooting step you can take if you have not done so is to try another charging cable and brick. I’m not sure how the technician came to their conclusion but it’s possible they just didn’t want to deal with it and just told you they didn’t know what was going on. Not sure how they can rule out the charge port itself if they can’t get it to charge.