FYI,
If you can't get the online 90 day check-in to work, turn your VPN off and it will work just fine.
TLDR : Answer Summary
Users report that turning off your VPN can resolve issues with the 90-day online check-in in Thailand. Additionally, using certain browsers like Safari and Firefox helps, while Edge may not work. Ensure that auto-fill, pop-up blockers, and auto-translate are disabled, and be cautious with the TM6 card number format. Some users have opted for traditional mail to submit their checks due to recurring online issues.