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

@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.

I used the Deco X50 model's dual pack without any issues for 1.5 months. One morning, I woke up to find it couldn't connect to the network. I replaced both the modem and the Decos with brand new...