ATT send me a new router for my fiber. I've had my deco setup and working for years- no issue. Plug in the new router and speeds went from 984/984 to 90/80

What the heck do I need to change? I saw other threads say to change BGW320 to IP Passthrough - but when I do that connection does and I have no internet. 
 

im a definite novice but can at least get to menus. 
 

I also can not switch deco to access points bc I use them for parental controls

thanks!

Heather