When encountering issues that you cannot download the firmware from the cloud server on USG. Try below workaround to solve the issue.
First, go to the web console function in the device and log in with admin
Give in the command:
_debug capwap show inter-ap-fw-table
- Reboot the USG. (not always so easy to do in environments)
- Update the AP firmware manually.
Go to CONFIGURATION>Wireless>AP Management>AP Policy>Firmware Updating. Please change the updating mode as "Manual". Then apply the setting.