Need help. Just upgraded from Deco M5 to an X55 with AT&T Fiber. All went well except my download speeds are under 100 Mbps and I have a 500 Mbps plan. Speed from AT&T to modem are 600+ but download speeds range from 70 Mbps to below 10. I have used several guides about setting the AT&T BG320-505 modem to passthrough mode but to no success. 
 

Any detailed instructions would be appreciated