The hub you have is called the V3 or the 2018 hub. There are some features, including setting it up, for which you have to use the new app.
It turns out that changing that particular model from one Wi-Fi network to another Wi-Fi network is just both complex and tedious because of the cloud component. You have to get the hub to disconnect from the SmartThings cloud long enough to register as off-line in order to make the change. See the following thread, which has contributions from several smartthings staff members.