To do this - you only need to update the field called 'APN' - please leave all other fields as they are:
Open Settings
Tap Mobile Service / Mobile Data / Cellular (The name varies depending on what version of iOS you have)
Select your Smart SIM
Tap Mobile Data Network
Replace just the top APN (Mobile Data) with data00.telnyx
Tap back.
Once done, it is recommended to restart your phone to reset your connection.