@JimBon
Hello, thanks for asking, please try the below suggestions:
1. Ensure the internet is working fine when the computer is connected to the Fiber modem directly, then disconnect the cable from the computer and put it into the main Deco M9.
2. Power off the Fiber modem and remove the coaxial cable from it (if it is a cable modem), wait for 10 minutes.
3. Turn on the modem and ensure it is properly restarts.
4. Put back in the coaxial cable.
BTW, who is your ISP?
Please refer to the below setup videos to reconfigure the Deco M9 to connect to the network if none of the above works:
https://www.tp-link.com/support/setup-video/
If it still shows failed to connect, please refer to the below troubleshooting and confirm which step did you get stuck with:
https://www.tp-link.com/support/faq/2239/