Im not sure it is...
Purpose of the backstop is something that needs to be in place - legally and operationally - by end of transition. At the moment that’s Dec 2020
The customs border, clearance hubs, trusted trader schemes, business readiness etc - you can’t do them in 14 months.
Given the EU want deal on fish before date to agree extension - no guarantees an extension would be mutually agreed
That means default date of transition end would need to be changed and extended.
And that means the two sides would need to agree what the UK would pay during that extension.