WiFi download speeds went from 15.5 mbs to 3.5. Other devices on same network still at speed. Have tried rolling back driver, didn't help. Surface 3 that I have had for 1 week. Any ideas?
Your Gateway seems to be using only WPA (hackable) while your extender is using WPA2/WPA mixed mode. You need to set your Gateway to WPA2 only and your extender to WPA2 only.
There are lots of complaints about the NVG510. Also I see that what you posted shows "66 leases" meaning 66 deices/computers have been connected. I'd make the WPA2 changes above and reboot everything.
Did this solve your problem?
Sorry this didn't help.