Cannot add back Arlo Pro cameras into Smartthings app

I have three Arlo Pro cameras connected to Smartthings. They have been working fine until the recent migration of Smartthings, in which my cameras are no longer integrated with Smartthings. After reading a recent suggestion that if I remove and add back the cameras, the Smartthings mode should appear in my Arlo account. I removed two of the three cameras but I cannot add them back now! When I tried to add the camera it directed me to my Arlo account and it stated that the device was not recognised. My Arlo account in my mobile is working fine and I can manually control all three cameras. I have recently changed to a new iphone, would it be the issue? I am no developer and would need someone to help me step-by-step:

(1) How to add my two cameras back to my Smartthings app?
(2) How to set up in Smartthings that siren will sound only on one cameras and send notification for the other two cameras when trigger?
(3) How to control Arlo Pro Cameras integrated with Smartthings with Smartthings triggers? Not sure if Arlo Assistant would solve this issue and yet it seems so complicated for me to copy/paste some coding into Smartthings IDE account etc. Thanks.