Shipping to custom address fields

I have created 2 custom fields in “Check Out form” and named them “District & Sub District”

These two fields have been adapted to show set values upon entry.

There are 12 Districts & 105 Subdricts.

In summary there is 1 District field with the 105 Sub Districts filtered against the District field entry.

This all works!!

However we are struggling to bring across the shipping zone costs to Checkout as this is defaulting to State and /or Post code.

We need it to look at Sub district.

Can anyone advise a solution.

Thank you