Just clicked on the link you provided. It didn't work, nor did going to the immigration site, clicking on "Online Services" and TM47 90-day Report, fails. Sure, the OP could use online reporting in some parallel universe, but the vast majority of the time since last November the site, along with the mobile app (which when it does work, wrongly shows overstay, inexplicably), is down.
In person works, I've never trusted the mail method, I've used a service in recent months, as going to Muang Thong Thani imm office takes the whole day going and coming and hundreds of baht transit, just not worth it.