The red circle shows the SIM Tray detector. The SIM Tray when inserted all the way pushes that copper bar down, breaking the connection on the left side of the SIM bay area in your Red Circle. When this connection is broken by the SIM Tray the iPhone knows to try reading the SIM Card. If the connection is complete (is not broken), like in the photo the iPhone will put up warning "No SIM card installed" and the top left corner screen bar will display "No SIM". If an empty SIM Tray is inserted completely and pushes the bar, breaking the connection, the reading begins, again there will be the warning "No SIM card installed" and the top left corner screen bar will display "No SIM".
When there is problems reading the SIM card, you can clean the SIM card metal contact on the card itself with isopropyll alcohol. If this does not help, visit Carrier and get new SIM card. Also reception problems can be due to antenna issues, make sure antenna ribbon is screwed to bezel at dock port correctly. Also make sure Logic Board copper pin at Do Not Remove Sticker is making good contact to bezel. You say the six copper Junctor Pins are fine, you can also clean these very carefully with isopropyll alcohol.