What happens if I'm late for my online 90-day check-in on a marriage visa in Thailand?

If you submitted your 90 day report online and they accepted it then you should be fine.

You can do 90 day report up to 14 days ahead of the due date and 7 days after the due date without any penalty.

Normally you can't do the online one late but have had a few reports of people being successful. If they accepted yours and sent you verification then you're good to go.

If not and you're still within 7 days you can go do it in person.

If past that then completely ignore until next time you go for a visa extension because the fine is 2000₿ whether you are 2 weeks late or 8 months late