@Checca 

When the Deco has a red light, please disconnect the Deco and the front modem, connect a PC to the front modem directly, do you have the internet on your PC?
If no internet access, it may be that the front modem does not provide a stable connection to the Deco thus Deco shows a red light, you can try to reboot the modem in this case.
If the modem is working fine, you can try to reboot the main Deco and check if the internet will recover. And you can try to use another Deco to be the main Deco.

https://www.tp-link.com/support/faq/2400/