I want to create query that reads a variable from the first
underscore to the second.

Maybe
M_ABC_123 = ABC
M_QWERTY_321 = QWERTY

I have writen this query

SELECT SUBSTR(partno,ATC("-",partno,1)+1,ATC("-",partno,2)-ATC("-",partno,1)-1) from plan

Its returns
ABC
QWE

What is the problem.
Thanks.