Re-enable Wi-Fi on a Time Capsule
You can also use Ethernet to connect to a Time Capsule with Wi-Fi disabled.


Connect Time Capsule and Mac via Ethernet cable.
Launch Airport Utility. If Wi-Fi on the Time Capsule is disabled it will show up with a yellow warning sign. Clicking on it only offers to 'Forget this device'.
Click on 'Other Wi-Fi Devices' in the top left corner and under 'Network Interfaces', choose 'Ethernet'.

The Time Capsule should appear. Clicking on it now allows to change its 'Network Mode' by first hitting 'Edit', then choosing the 'Wireless' tab, followed by selecting Wi-Fi from the dropdown menu. Applying the settings will take a few seconds and require a restart.