Yes. I remembered having this issue also. Very frequently.

what phone are u using?
do u get dropped calls when using VoWiFi?

how long is the digi prepaid grace period?

so let's say my im in grace period. on the 90th day prior expiring, i purchase the rm 1 validity. so i'll have another 90 day grace period correct?

that's a cheap way to maintain your number when not in malaysia.
purchase every 2 month should be safer.

through dealer meaning need to get new simcard?

you will get a sms for a start once your validity expired.

@tympg gave another better solution if u forget the date or lost the sms. try to put in your calendar and set a reminder 2 weeks before expiring and then 1 week before expiring.
best is to purchase validity every month. RM 1.