X7 No Connectity via Ethernet devices but WiFi functions normally

I have an X7 gateway (Arris TG4482A) serving as the router and Wi-Fi access point for approximately 30-40 devices at any given time (and around 110-120 different devices over the 7 months since it was installed).  The devices connect through a roughly 50/50 mixture of Wi-Fi and Ethernet connections. The Ethernet connected devices are all run through unmanaged gigabit switches in 4 different rooms, each of which is connected to a hub switch. The hub switch is also connected directly two devices and the gateway (which has 2 devices directly connected to it).  The gateway is set up with default settings — it manages DHCP (IP addresses 10.0.0.2 through 10.0.0.253) and the subnet is 255.255.255.0. 

This setup functioned without any issues until last week, when suddenly (almost) all of the ethernet connections stopped being able to connect to the internet or see other devices in the network but the devices connected over WiFi all continue to function normally without any issues. Only those devices directly connected to the hub switch or directly to the gateway can function over Ethernet.  And even then one device connected to the hub switch does not work. No new devices were added to the network. 

Looking at the AppleTVs which are connected via Ethernet I see that the gateway is putting the device on a different subnet 255.255.0.0 and assigning an IP address of 169.264.20.2XX.  These devices don’t have internet access and do not show up on the gateway as being connected — despite all showing an active Ethernet connection.

Customer service suggested my x7 cannot handle the number of devices, but I am nowhere near the client limit.  I’ve tried swapping out the hub switch and well as the switches in individual rooms without any luck.

I have the same setup in another home but using Ubiquity/ UnFu hardware and the x7 in bridge mode without any issues.   

My initial  guess was a bad switch(es) or a rouge DHCP sever but I haven’t been able to identify either. Since this setup worked for 7 months without issue it seems like a hardware failure — but I know just enough networking to be Dani so it’s also possible/likely I’m missing something obvious.