Is it possible to turn off the 1.8 GHz transmitter on an iPhone via software?
I would like to use WiFi calling only.
No. Apple doesn’t allow control over specific radio frequencies. You could jailbreak and try installing / modifying the baseband software or avail yourself of the normal controls to turn on or off WiFi, cellular data, airplane mode and Bluetooth. Note, turning many of these off doesn’t disable the radio, but disassociates them from the active connections. Based on what you intend to do, that might suffice.
Keep in mind, you don’t need a SIM or even a registered phone to dial 911 (in the USA and other emergency numbers globally) so the radios stay running independent of the cellular data and service working.