Since Digi/Celcom merged ;they've streamline the process. Previously, once I purchased credit top up it auto extend credit period; then I have to wait until the credit period to expire; only then purchase SLL.
Now I just top up using T&Go ewallet app; on the day BOS offer available ,then purchased the discounted SLL offer.; Digi App shows it send SMS/but no SMS) but  it's good to go;when  DIGI app upon refresh will show new (1 year) credit validity date

Ever since celcom-digi merger, I found unifi mobile to be better coverage in places I frequent in JB. Not sure  about NS HIGHWAY.

I just simply chose the 1st option when I purchased the Internet plan,
which is NEXT 20 (Auto-renew). I just subscribed because I want to make to make the RM38 plan appear in my box of surprises.
It is just for one time, after that I just unsubscribe the plan.
So actually I have spent RM58 (RM20 the Internet plan + RM38 Super Long life) to extend for one year.

The RM38 will only appear in your box of surprise from the 15th of the month until the end of the month.