How I did it was that I have the guest SSID tagged to the guest VLAN. Then in my router, I have assigned bandwidth restrictions and priorities based on the VLAN. Ie. the guest VLAN has a low priority, and total bandwidth is throttled. I'm only using one router.
You said that the 760 can ping the router, but can the guest clients ping the router? Is the LAN port connected directly to the router, or is there a switch or anything in between?