Some Samsung Galaxy Note 8 users cannot connect to mobile networks after installing the July 2020 Android security patch. The phone refuses to detect a SIM card outright. An OTA update is required to fix the issue as standard troubleshooting steps don't seem to work. Samsung is yet to acknowledge the problem officially.
https://www.notebookcheck.net/Some-Galaxy-Note-8-users-are-unable-to-connect-to-mobile-networks-after-installing-the-July-2020-Android-security-patch.483381.0.html