What should I do if I am overseas on the 90-day reporting date for a Non-O Retirement Visa in Thailand?

Nothing. You arent in the country. The 90 days is to report 90 days in country.

Just count 90 days starting the day you arrive back and are likely to have to do it in person.

You can't be "out of the country" and have a 90 day report due

Here's how 90 day reporting works:

If you're on a visa/extension that does 90 day reporting and you're in country 90 continuous days you need to file a report.

If you leave the country on or before the 90th day the "count down clock" stops and when you return to Thailand your next 90 day report is due 89 days from the date you stamp in. (The day you stamp in us counted as day 1).

Your first 90 day report after coming back into the country usually has to be done in person.

You can file a 90 day report from 2 weeks before the due date until 1 week after the due date with no penalty