Numbers 12 does not recognise custom date format for timestamp

Yes, because Custom Data Format can control how a date-time can be displayed, but it can't effect the actual value in the cell.

So you need to convert the ISO-8601 to a format Number recognizes, such as yyyy-mm-dd hh:mm:ss.

If you just need the Date and don't care about the Time then this will convert a value in B2.

=TEXTBEFORE(B2,"T")+0

If you need the Time as well then you can try this:

=(TEXTBEFORE(B2,"T")&" "&TEXTBETWEEN(B2,"T","."))+0

The equivalents of these with Language set to French and Region set to France would be:

=TEXTE.AV(B2;"T")+0

and

=(TEXTE.AV(B2;"T")&" "&TEXTE.ENTRE(B2;"T";"."))+0

You will lose the milliseconds.

You may be wondering about the +0 in the formulas. It's just a trick to force Numbers to treat the result as Date-Time (right-aligned by default) rather than Text. You can then reformat the Numbers if you want in the pane at the right.

Once you have the dates in the proper format in Numbers you can "remove" the formulas by selecting the cells, typing command-c to copy, followed by Edit > Paste Formula Results.

SG