Reminders for custom-repeated transactions

If I choose Custom repetition interval (for example, Daily + every 30 days), I can’t set reminder at any time other than “On Due”.

What I want is to still be able to set it as “1 day earlier” and so on, which I could do, if I set the repetition interval as “Monthly” (but it’s not working for here me as I need exactly 30 days, not 28-31, so not a workaround).