According to
page 3 of this document Jersey Telecom has an roaming agreement with Digicel, so UM+ should work down there.
So can you register on that network? If only callbacks fail, try triggering by USSD: Just enter *137*phonenumber# (e.g. *137*442079765722#) followed by the pick-up-key. Beyond that you could also try sending a text so see, if communication with UM+ fails completely or not.