Can I do my 90-day report online or do I have to go to immigration?

You can usually file the 90-day report online through the immigration bureau website or their app, and there is also a postal option. Many people still do it in person at Phuket immigration because the online system can reject reports if the timing window is off. Try online first (it opens 15 days before to 7 days after the due date), and keep the confirmation.