This is a older generation router and the CPU and HW is probably not able to handle the 900+Mb ISP speeds. Others with Rev As have also reported this. Also the DGL-4500 and I presume other routers of this era have the same limitation. Most ISP speeds of this era wasn't much over 100Mb. I was still using 6-12Mb on cable. I got a good increase back in 2008-09 to 50Mb with my ISP. Then two years ago we got 100Mb. My ISP just started supporting 1Gb speeds in certain areas where I live. So most DIR series routers of 10 years ago or more are just not able to support 300Mb and above ISP speeds.
Only thing I can suggest is to use the ISPs ONT/router as the main host router and use the 655 as a LAN to LAN wireless AP. You'll only see about 300Mb on wireless which what the 655s only the connection rate these router can do. They make good APs for remote locations in the house.
If you want to use a different external router, then I recommend getting into a newer generation DIR series router.