Are you checking a bag? You will need to collect your bag from your United flight and then go out and back through security after checking in for your Intercaribbean flight. Intercaribbean’s counter opens 1.5 hrs prior to departure, maybe less - 2 hours should be okay if your UA flights lands on time.

I have flown them a few times now with no problems. Make sure you know what their policies are around COVID.....