Very slow 2.4 Ghz connection with AC1600 WiFi Cable Modem Router

I installed the AC1600 WiFi Cable Modem Router to replace my Comcast wireless gateway.  Previously I would get over 100 Mbps on the 2.4 Ghz connection with the Comcast gateway but now I get between 5 and 15 Mbps with the new router even when connecting 5 to 10 feet from the router unobstructed.  The 5 Ghz connection works fast with over 100+ Mbps even up to 30+ feet away.

I have tried to change from Auto to channel 1, 6, and 11 but that fix the problem.  Does anyone know any solution or other things to try?  Thanks!