I have an S2120 8in1 LoRa weather station (868MHz) on TTN in unconfirmed mode (1N). It worked perfectly fine until I deleted the device in the old application and tried to add it to a new application. I cannot physically access the device, otherwise it would work for sure.
Can the device reconnect at some point?
Only if device automatically tries to join again by itself after a while ! If not your only solution is to access physically the device unhappy 
when does the device try to rejoin again?
Some small devices like sensors are designed that way but from what I can see for the S2120 Join command has to be triggered from the mobile app so you’ll need to get physical access at device to get it to join again the network
It may be difficult for the device to reconnect automatically. This is because LoRa devices typically use device-specific keys and other information that is tied to a specific application and network server. When you delete the device from the old application, the device-specific information may be removed from the network server, which could prevent the device from reconnecting to the new application automatically.