Ok, just to review and make sure I understand what your setup is, which I believe is:
CGN3 (in bridge mode) connected via ethernet cable to the Belkin router which is running in full router mode (firewall, packet inspection, port forwarding, etc, etc.)
The CGN3 port LED shows amber, which means that the CGN3 to Belkin interconnect is running at 1 Gb/s.
What level of service are you supposed to be at, and what are your speedtest results via wired connection through the Belkin to a pc or laptop?
Ok, I see your Macbook plugged in shows 58 Mb/s.
One of the problems that people are running into these days is whether or not their wired and wireless devices are able to use the higher data rates. The only solution is to review their specs and settings in conjunction with the router specs and settings and determine the best compromise for everything that you want to use. There might be faster settings available, but until now, they may not have been needed. You might also find that some of the older devices simply won't support higher data rates.
Ok, stupid question just to make sure......when you bridged the CGN3 you went into the Gateway Function tab in the CGN3 and disabled the "Residential Gateway Function"? At that point after saving the setting, the CGN3 should have rebooted into Bridge mode and you should not have any Wireless transmission LEDs showing at the the bottom of the front LED display? If for any reason you were still in Gateway mode the 2.4 and 5 Ghz default networks on the CGN3 would be up and running, providing interference for your Belkin network. That would result in low wireless rates on the Belkin network. As I said......just checking.
The CGN3 in bridge mode acts solely as a modem. Cable data in, ethernet data out, and vice versa. Thats all it does. That means that the Belkin has be running in full router mode with the firewall enabled, deep packet inspection enabled if you prefer and any port forwarding as well would have to be done by the Belkin. Most new routers these days come with different operating modes available. You can run them as a router, a wireless bridge, an Access point and possibly others. So that requires a read through the manual, in the appropriate section, to ensure that everything that needs enabling for a specific operating mode is in fact enabled. Each router will have its own specific settings for the various modes although for a full router mode, most of them are common across all brands.