Just to update this, extending the lifetime of a DTAC prepiad SIM is now *113*Number_of_Days# (where Number_of_Days can be 30, 90 or 180)
Alternatively, on the www
https://app.dtac.co.th/en/home?passive=y&utm_campaign=home_accounticon&utm_medium=web_owned&utm_source=webdtac
(Use any network to get to the webpage. Login with your phone number. They will text a OTP to your DTAC phone, enter that password into the web browser, then follow The Menu path:
Beyond > Jai Dee > View All > Day Giveaway