@marla2394
Hello, the Deco E4 supports ethernet backhaul and wireless backhaul, in other word, the Deco units can connect to each other via wired cable or wireless.
The powerline backhaul is different from the powerline adapters, we do able to use the powerline adapters to boost the signal.
If the ISP modem is a pure modem, it is suggested to connect the Deco E4 to the modem directly and configure it as a main router, then you can connect the old powerline adapters and the slave Deco to it to build up the network.
Or if the ISP modem is a modem router and you have located them like the layout attached, it is suggested to configure Deco E4 in access point mode.
May it help and have a nice day.