Finally a reply from Rain
"Note that all Rain-Fixed LTE will receive double their monthly allocation effective 1 December 2018, this is regardless of ISP as this promo will apply to everyone. This bandwidth will be divided between peak and off-peak. Carryover rules will only apply for Peak DATA, any unused off-peak data will not be carried over. Should you deplete your peak DATA before the off-peak data, you will then be capped during peak periods and your services will only be accessible during off-peak times. you do however have an option to top-up in order to continue using during peak periods. should you deplete your off-peak DATA before your peak DATA, you will not be capped and the service will carry on and deplete from your peak DATA bucket."