The SB 6121 should work well as I have used it on my DIR-657 router, same class router as the 827.
I have a 6180 now,
Check the cables to be sure, preferred is CAT6. Some cables are made cheaply.
Ensure the Setup/Internet/Manual section is set for DHCP on the router.
Power off both modem and router. Power ON the modem and let it sync, the green globe light should be lit. Power ON the router and the Link light on the modem should light up amber or blue if your using the Gb connection and cableing thats good.
Might try to change the routers WAN port speed settings to 1000 and test. Try setting other speeds as well. Possible that the router could be bad. Lets find out.
Keep us posted.