D-Link DIR-2660 rev A1 Not Working After Successful Upgrade

Thanks Pico,
Probably I have to tune WiFi hard way to get reasonably good internet speed for clients at least not lagging for the speed I’m paying.
If I connect laptop to LAN port of main isp router , speed test shows between 800-900 Mbps.
I couldn’t get even half of the wired speed for WLAN clients.
What are more powerful HW available, so that upgrade could be considered.
You know these days work and schooling is behind these home routers. It looks like router is clogging.