90-day reports are currently processed at the Muang Thong Thani immigration office, not at Cheangwattana. Also, the appointment system is reportedly not functioning, so it may not be necessary to book an appointment.
90 DAY REPORTING RESOURCES / SERVICES
Use the trusted Thailand 90 Day Reporting Service to get your in-person report done and mailed to you for as low as 375 THB (even if the online system doesn't work for you).
For immediate assistance, contact Thai Visa Centre directly via LINE at @ThaiVisaCentre or Email them.