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Hi and welcome to the site, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially, bear with us; re-confirm and post back with the complete model number of the computer and the modem/router.
Appreciate the additional information.
On "... computer is not showing a wifi app...", the Dell OptiPlex 755 does not have hardware provision for WiFi; it does not have a WiFi card.
Based on its Technical Guide Book, it only has an Intel 82566DM Gigabit LAN 10/100/1000 integrated on the system board on most versions or in some others, a Broadcom NetXtreme 10/100/1000 PCIe card.
If that's your computer; Dell OptiPlex 755, then yes.