What should I do if I don't receive my 90-day reporting receipt before the due date in Thailand?

That is not an extension. That is just a longer window to submit 90 days report in person.

90days report windows are:

- At the local immigration office: 15 days before until 7 days after the due date

- Online: from 15 days in advance until the due date

Immigration offices are closed until today for long holidays, so maybe wait for this week. If you get your report rejected, you may be able to try online once more, or just go to the immigration office to submit.

If you don't get any results but you want to do it in person, make sure you "cancel" your pending report application before you submit one in person or tell the officer you have a pending one online, so that you don't have to mess up your reports both online and in person.