|While it's nice to have the higher-end hardware, for instances like this I'll use my cheap physical hardware model...|
I'm assuming right now you've got the switch plugged in directly to the Ethernet port on the Adtran (not really familiar with this device). In this case, I'll assume it is already doing NAT. I'll also assume that the Adtran NAT's to 192.168.1.x
You'll need to buy 2 cheap wireless routers of your choice, and possibly 1 new switch depending how many devices you have on the business side
Wire and configure as follows (all subnet masks are 255.255.255.0):
Switch 4-port (or more)
Cheap Wireless Router Cheap Wireless Router
(Business Side) (Wireless Coffee Shop)
(Set WAN 192.168.1.11) (Set WAN 192.168.1.12)
(Set LAN 192.168.2.x) (Set LAN 192.168.3.x)
(DHCP On) (DHCP On)
(If needed for more PC's)
Configured in this way, the Business side devices will be on the 192.168.2.x subnet, and the coffee shop side will be on the 192.168.3.x subnet, but both will access the Adtran on 192.168.1.x subnet.
And yes I realize it is double NAT'ing, but this is a cheap and easy real world solution I use for clients that can barely rub together 2 nickels, and it protects their networks.