@alexvdv,

There is no way to configure this properly; you can only give your network its best shot for changing its connection. Make sure that you have configured your network using dynamic IP, as this should allow the IP address to change.

After talking with my team, if your situation fits the bill perfectly - you may be able to actually connect both connections to the main Deco. You cannot choose which would be used for the active WAN as when one WAN ethernet connection is found, the others will turn to LAN connections. 

It is also likely that you would still need the DHCP leases to expire, so you may have to reboot the Deco or wait an hour for the Deco to reconnect with the other WAN.

Without extra hardware, your best option is really to manually perform the reconnect.

  @alexvdv,

I mean, once you add a router above the Decos they will need to be configured in AP Mode rather than as a router. In AP mode, they are a much simpler network and will work with most any setup. 

The ER605 is popular as i see a lot of users with it, however we only deal with consumer-level products so i cannot provide the best recommendation. 

You may also consider that a simple switch between your modem and Deco may be able to 'turn on/off' a specific ethernet(WAN) connection, we don't quite know how the Deco will handle the transfer, but it may allow for you to make changes on demand.