First, there is a specific subforum for the EX2 Ultra. This subforum (My Cloud) generally discusses the single bay/single drive My Cloud unit which is different than the EX2 Ultra. Single bay My Cloud units are not supported by OS5.
One does not need to use the online mycloud/setup method to setup a My Cloud device. Instead one can access the My Cloud Dashboard and perform the various steps needed to configure and setup the device. Several articles about the My Cloud Dashboard and using the Dashboard to setup a My Cloud device can be found at the following link:
Make sure to disable the automatic firmware update option in the My Cloud Dashboard > Settings to avoid having the unit update to OS5.
One can download and read the OS3 User Manual for the EX2 Ultra at the following EX2 Ultra support link: