There are no direct flights to Montserrat. You fly into V.C. Bird International Airport in Antigua first, then take the short hop to John A. Osborne Airport or the ferry across. That layover is the perfect time to have your eSIM ready, so buy and install it before you leave home while you still have steady WiFi.
Installing is quick. After you buy a plan you get a QR code by email. On your phone open Settings, then Mobile Data or Cellular, then Add eSIM, and scan the code. Give it a clear label like Montserrat FLOW. Do not turn data roaming on yet. You just want the plan loaded and waiting.
Check one thing before you pay. Your phone must be eSIM capable and carrier unlocked. Most iPhones from the XS on, recent Google Pixels, and newer Samsung Galaxy phones work fine. If you are not sure, search your model plus the word eSIM.