Dbarunas,
You left out some important details, but let me see if I can answer without them:
Assuming that the ISP modem you mentioned is your current wifi destination and the Airport card is the Mac Pro built-in wifi adapter. I am also assuming a desktop computer not a laptop since you didn't mention MacBookPro.
1. If your router is wireless and assuming the "airport card" is the computer built-in wifi adapter.
modem (wired) -> (wired) router* (wifi) -> (wifi) laptop ( * hardwired to printer) Connect to the router via wifi instead of the modem, while disabling the wifi on the modem once the router wifi connection established, if possible. (No need to nuke the family bits with too much wifi signaling.)
2. If your router is not wireless.
modem -> (wifi) laptop* (wired) -> (wired) router (wired) -> (wired) printer ( * Make sure the computer does not have a gateway specified in the network settings for this static address assigned connection.)
3. If your router is not wireless and no other hosts on router. (no router needed)
modem (wifi) -> (wifi) laptop (wired) ->* (wired) printer (local static addresses only, again no gateway) ( * this may require an Ethernet crossover cable depending on the printer capabilities.)
Where you are using a wired network on the laptop and wireless to Internet, make sure the "backend" wired network is on a completely different range of addresses and 'NO' gateway assigned or the laptop will not know how to go out to the INET.
Joe