@HFAHFA 

I assume you're using Deco mesh in Access Point mode, as indicated by the AccessPoint tag in your post. 

This network configuration should function in Access Point mode provided the ISP modem/router is compatible with the Deco Ethernet backhaul feature, which relies on the standard IEEE 1905.1 protocol.

You mentioned that this configuration previously worked. If it's no longer functioning, it could be due to a firmware update from the ISP to their modem/router, which may have unintentionally broken the support for the IEEE 1905.1 protocol.

It's possible that your ISP already has a firmware fix. The simplest way to verify is to turn off your modem for 3-5 minutes and then turn it back on. During the power-on process, the modem should automatically search for any new firmware updates from your ISP.

Ensure you perform a hard power off by unplugging the modem. A soft power off, like a restart, may not prompt the modem to check for firmware updates.

If after that your network setup still does not work, consider your modem as (currently) not supporting Deco Ethernet backhaul protocol.

  @Alexandre. 

Thank you, I will use it in the following configuration until the modem's next firmware update:

Modem -> Ethernet -> Deco 1 -> Ethernet -> Deco 2

This has fixed the situation for now.

Yes, the problem was stemming from the modem. TP-LINK LS1005G switch solved the issue.