It depends what you are trying to do - whether its the samatha or vipassana variant of anapanasati. So, this is dependent on what you're doing.
Don;t be too alarmed by the breath becoming too subtle to observe. Its still there but in the beginning our powers of perception are not great and, in time, we're able to discern subtler and subtler sensation for longer and longer periods uninterruptedly. The samatha variant of anapana I practice (from time to time), I maintain awareness of the touch of the breath under the nostrils. If I were to lose awareness of the touch of the breath, I exert a couple of forced, harder, breaths, to reorient my awareness, to anchor my awareness before returning to natural breath.
If your intention is to practice samatha, then dispersing your awareness throughout your body may not be conducive to developing the sort of samadhi that is then useful in vipassana.
BTW, what instructions do you have with your meditation kit?
“No lists of things to be done. The day providential to itself. The hour. There is no later. This is later. All things of grace and beauty such that one holds them to one's heart have a common provenance in pain. Their birth in grief and ashes.”
- Cormac McCarthy, The Road
Learn this from the waters:
in mountain clefts and chasms,
loud gush the streamlets,
but great rivers flow silently.
- Sutta Nipata 3.725
(Buddhist aid in Myanmar) • •