Check Control Panel, System, Hardware, Device Manager to see if the wireless adapter is detected and if a fault is indicated. Next step may be to update the driver for the wireless adapter from the Acer support site.
If that doesn't work, you'll probably find it easiest and cheapest to add a USB wireless adapter.
Fihrat, I see Network adapters which has three adapters: Broadcom 802.11g ..., Cisco Anyconnect...., and SiS 900-Based PCI... Are these the wireless adapters. If so, I guess the second step is to check for update for the driver. By the way, my other laptop does work with wireless modem.