Under iMessage settings there is no text message forwarding

I am trying to setup text message forwarding, as per this support article:

https://support.apple.com/en-gb/HT208386

However, on my iPhone, Settings, Messages, there is no "Text message forwarding" option listed.

iPhone XS Max

iPad Air

The "Reachable At" phone number listed on my Apple ID Account settings is that of my iPhone.