score:4
Just call now (or as soon as you can) and confirm your appointment for date and time, and tell them you will be hiking and will be out of range until that date.
I don't see any problem with that.
Upvote:2
It might lead to cancellation of your appointment, but if you notify them in advance that you will be incommunicado then you will almost certainly be able to avoid that outcome. In other words, the cancellation should not be automatic and unavoidable.
Source: I did this once for a dental appointment.