RT/tty button

So, you're saying that if you go into Settings, there is no "Accessibility" between "Home Screen" and "Wallpaper"? If so, there is something amiss with your phone. Try going to Settings>Screen Time and seeing if there are any restrictions enabled.

"RTT and TTY aren’t supported by all carriers or in all countries or regions. RTT and TTY functionality depends on your carrier and network environment. When making an emergency call in the U.S., iPhone sends special characters or tones to alert the operator. The operator’s ability to receive or respond to these tones can vary depending on your location. Apple doesn’t guarantee that the operator will be able to receive or respond to an RTT or TTY call."

Set up and use RTT and TTY on iPhone - Apple Support