I previously activated Secure2u on the MAE App, but uninstalled it and then recently reinstalled it on the same device. Do I need to reactivate Secure2u?

Yes. You will need to activate Secure2u on your mobile device again. However, you do not need to activate it through the ATM and wait for 12 hours to start approving transactions, as you have previously activated Secure2u on the MAE App on the same device.