For context, Frontier Airlines' parent company Indigo Partners has ordered 32 

 planes, with Frontier to receive 18 of them, scheduled for delivery in Q2 2024. However, the plane's delivery date has been pushed back, originally planned for late 2023. The A321XLR completed its 

first transatlantic passenger test flight

Furthermore, Frontier hasn't definitively stated whether or not it will fly to Europe:

https://aviationweek.com/air-transport/airports-networks/frontier-airlines-ceo-transatlantic-a321xlr-routes-definitely

Frontier would only be able to fly between the northeastern US and Europe. Its parent company also owns Wizz Air, a European low-cost carrier.

https://thepointsguy.com/news/frontier-airlines-europe-barry-biffle-interview-a321xlr/