Your first 90 day report will be due 90 days after entering Thailand or on the date provided by immigration when you did your last 90 day report. Doing a extension is not a 90 day report. Only if its the first time after a 90 days visa.
Only saying that not all treaties are the same. And in the old treaty there is also a source priciple but was limited to governement income/pension. And thats what i have. So no taxes in Thailand. Cheers