Installing JGS524E - can only see switch on network, not other computers or Internet

I have a newer cable modem connected to a ProSafe VPN Firewall FVS338 which acts as my firewall and DHCP as well as handling port-forwarding. I had an 8-port ProSafe GS108 switch connected to the FVS338, and then my computers, etc connected directly to the GS108 which all worked great for years. Needing more ports, I just purchased the 24-port JGS524E to replace the GS108. When I unplug everything from the GS108 and plug everything into the JGS524E, I can see the JGS524E on the network via the ProSafe Setup utility, but I can not ping or see any other device on the network (including the FVS338) other than the JGS524E. I have upgraded the firmware on the JGS524E to the latest version 1.00.21, have tried factory defaults, moving the cables to different ports, etc. I've tried the JGS524E in DHCP mode (it does get an IP address from the FVS338) and with DHCP disabled, manually assigning it an IP address on the same subnet.

When I power off the JGS524E and back on, I can BRIEFLY get an IP address via DHCP on the FVS338 from a connected computer AND get to the Internet through it. But it dies within a couple of minutes.

I basically just need the JGS524E to act like a "dumb" switch: computers/devices can see each other on the network, DHCP is passed through from the FVS338, and everything can get to/from the internet, and for more than two minutes. What am I missing?

Thanks in advance!