Changing number of posts per page on CPT-archive.php, have tried 20-30 code chunks so far

Using the Query Monitor suggested by @Andrew (Thanks, Andrew!) I found a little gem in my parent theme that overrides everything set in my function.

I found out wordpress loads the child themes functions.php first
myfunction{
$query->set( 'posts_per_page', 15 );
}

so when it later loaded

lameparentfunction{
$query->set( 'posts_per_page', 5 );
}
It undid the changes I did.