How do I get my controller back online

Try rebooting the controller (unplug AC power, wait a few seconds and plug it back in). If no luck, see

or

for ways to reconfigure Wi-Fi without a smartphone.

If no luck, ask a friend or neighbor loan you an old smartphone they likely have lying around. It doesn’t need phone service or even a SIM card, as long as you can connect it to Wi-Fi to install the app.