Hi,
I've just tested multiple scenarios of your described issue but I was unable to reproduce it. Both the Setup Wizard: Mobile window and Scan for operators work as expected with RUT9XX_R_00.05.03.3.
Did you upgrade with the "Keep all settings" option? Perhaps there is something wrong with the configuration and it carried over even after the upgrade. Can you try resetting the router and reconfiguring it?
If that doesn't help, can you provide the router's Troubleshoot file? It can be downloaded from the router's WebUI, System → Administration → Troubleshoot section. You may want to remove any confidential info from your configuration before you download the Troubleshoot file. Usernames and passwords are blurred out automatically, but things like phone numbers are not.