There are a few simple checks to make before trying to learn
your keyfob.
Are you close enough to your caravan? The range of the keyfobs
is 6M, try moving closer.
Does the torch work? If not, the battery may be discharged and need replacing.
If the torch does work, does the red light come on when the arm/disarm
button is pressed? In this case the keyfob
is likely faulty and should be replaced.
Does the other keyfob operate the alarm correctly? If not, confirm that the alarm has power
(does the green light come on when you move around the van?) and ensure power
is available before trying the keyfob again.

If the keyfob still doesn't operate the alarm, then you will need to relearn the keyfobs, the relearn instructions can be found in the product page here: AS300 - Stinger Alarm System