@Stefan1977 

The failover means when one WAN port fails to work then the second WAN port will take effect. This is a function in the "Link backup".

But for the Deco issue, you can try to change the operation mode into access point and monitor. 

When the Deco has no internet, what is the IPv4 status on the APP? At the same time, you can try to replace a PC to the Deco back of R470T+ and take a look if the PC can get internet from R470T+ when there has two modems in front of R471T+

@Stefan1977 

It looks like the R470 is not able to detect the WAN active status. This could be due to the ISP routers act as the WAN for the R470 (Double NAT)  Can you share more details about your connections. 

1. What is the Connection type you have selected for the WAN ports in R470. (Static, Dynamic, PPoE)

2. What is the actual WAN type (Fiber, ADSL etc) and what kind of router device that you use for connecting to the internet ?

The easier approach is to configure the link status detection to manual mode and configure appropriate details for the same. 

Advanced approach is make the R470 to connect to the ISPs directly using PPoE, if your ISP supports.