In my case the Rollover data was used up before the Offpeak data during both Peak and Offpeak times. I currently have no Rollover data left, 512MB of Offpeak and my complete 25GB allocation of Peak data remaining. I guess I am about to discover shortly whether the Peak data can be used during Offpeak times which I am hoping is the case. Did you possibly use up all of your Offpeak data at the start of the month before the Rollover data had been allocated?
Edit: On second thoughts maybe the Rollover data is only consumed during Peak times? Keep us posted if your Offpeak access gets cut off.
For some reason Rain seems more interested in attracting uncapped mobile customers than in retaining their more lucrative Fixed LTE customers with more generous data allocations.
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.... (although must 'fess not quite the same league as the 1Tb'ers )