Computer Does Not See Modem When Using Ethernet Switch

Back story ... I build control panels. My customer wants one that can be controlled via the internet. So, I have a PLC with an Ethernet port, a touch panel with an Ethernet port and a cellular modem with an Ethernet port. I also bought a "dumb" Ethernet switch. The modem, PLC and touch panel will all plug into the switch.

The PLC will be used for controlling and sending out e-mail alerts.

The touch panel has a built in program that allows it to host (well really it fakes it) an web site (really, it just lets you duplicate the touch panel on a web page).

The PLC and touch panel should have no problems talking to each other since they are from the same supplier (Automation Direct).

So, first problem ... unplugged the Ethernet cable from my computer and plugged it into the cellular modem. Typed the address given to me in the manual in Firefox and got to the modem with no problems.

Then I added the Ethernet switch ... my computer to the switch to the modem. Now I can't talk to the modem ... why?

I know the switch works (tried it between my computer and my cable modem ... works fine).

Since it was a dumb switch I figure putting it between the computer and the modem should have been a no brainer ... what am I missing????

Using a cellular modem is the only part of this project I have not done before ... flying by the seat of my pants here. I am sure I will have a lot of learning to do in a hurry. Not even sure how to set up a network between the PLC, touch screen and cellular modem (windows seems to do all this by itself).

Thanks .... Mike