Follow this guide to replace the RJ45 door bracketflap in your Lenovo ThinkPad T16 Gen 3 laptop.
A flap that's in need of replacement may no longer hold an ethernet cable in place, or has simply broken off.
The Lenovo part number for the RJ45 door bracketflap assembly is 5CB1M21645 for the WLAN model or 5CB1M21651 for the WWAN model. This guide features the latter, but the process is identical for the former.