Hi @successchr21051,
Has the phone been dropped or gotten wet at all?
Have you tried inserting another, same size, known working, network compatible SIM card in the phone to check if that worked OK as we don't really know what you've tried?
If the answers to the above two questions are No and Yes then here's the Galaxy S6 [https://elektrotanya.com/samsung_galaxy_s6_g920f_service_manual.pdf/download.html|service manual] that may help.
It shows how to disassemble the phone, has fault finding flowcharts e.g. the SIM troubleshooting flowchart is on p.8-14, board component layout images showing the location of the parts mentioned in the flowcharts and a parts list.
Hopefully this is of some help.