I have a Galaxy S20 FE 5G (ported from Verizon) but Wi-Fi Calling is not in the settings

WiFi Calling for Samsung devices is firmware dependent.  It doesn't matter what carrier the device was bought from in the US, Canada, and even with some overseas models.  Verizon and AT&T use proprietary versions of WiFi Calling that is different from the stock version of Android implementation, which T-Mobile uses.  If you want WiFi Calling to work on T-Mobile for your Verizon sold Samsung device.  You are going to need to flash it to the U1 firmware, which will adapt to the SIM inserted in the device.  You can go further and flash from the U1 to the TMB U firmware to get all of the T-Mobile bloat and a couple of different service icons but ultimately, your experience on the U1 will be pretty much on par with the TMB U firmware.