Intel(r) 82578DM network card problem

Hello, I can have the internet connection speed of only 40mbps max (ISP declared speed is 500mbps) with wired connection but not wifi. My network card (Intel(r) 82578DM) supports 1gbps. I can choose 1 gbps from drop-down menu but if I confirm it at 1mbps the internet connection is lost. I thought it is limited by BIOS but can't find anything about the speed limitation. I have Dell OptiPlex 980. Service Tag: with Win 10. Please help me to fix the issue.