specifically wrote "So if you recently re-entered the country you need to do the next 90 day report *in person*". This is also what the rejection email says if you submit online.
I've seen reports of people submitting their 90 day report after re-entering the country by mail. Anyone authoritatively know whether this is possible at Chaengwattana?
Definitely get rejected in my experience. And if you wait until 15 days before due, you almost certainly wouldn't get an appointment at Chaengwattana, so might have to hang around pointlessly for hours.