Hi @Natthawut, a flashing downstream light is an indication that your device is not getting a signal.
For customers experiencing any issues with their NBN service, we recommend the best thing to do first is check if there's an outage on the NBN network.
We have created this article that can guide you on How to Check for NBN Network Outage
If there are no known outages in your area, we recommend checking the equipment set up: NBN Fibre to the Curb (FTTC) Setup Guide
If the equipment is properly set up, you can proceed with the troubleshooting indicated on this link: https://www.tpg.com.au/support/nbn-fttc/problems_connect_internet_nbn_fttc
In your case, your NBN FTTC service has not yet activated and this has been raised to NBN Co. to resolve your specific concern.