is the FLEX on your network? Try running AngryIP * or some network mapping program, some routers can show you active IP addresses others do not.?
ping in the command (terminal) box works as well. PC or RPI... Among other things you could have a bad cable, port,switch, even the FLEX itself... Let's wring out the hardware 1st.
* free download at:?
Regards,
Larry W8LM