I had a W2400 two unit setup (main and satellite) and recently replaced the main unit with a S4 to get the gigabit port, thus better WiFi speeds.

The main unit is connected to my router and is working on WiFi router mode. My router has WiFi disabled, so only feeds internet to the main S4 via ethernet.

All works fine, my devices connect with no issues, however the main S4 unit always has a solid red light on. I can see both units in the app, even when accessing from outside my wifi network. Diagnostics in the app all turn up ok.

I've seen other threads in here for same symptoms, but I can't find what else to check, and what does the red solid light mean in this case?

Thanks!