@Housearrest 

It will cause double NAT issue, but not every home network will be negatively impacted by that. For example, I know that on my home network double NAT does not do any harm.

If you find some equipment or software on your home network can't handle double NAT, search for "switching to bridge mode" instructions for your ISP router. They may be in user manual, or obvious from ISP router Web interface, or you may have to call ISP tech support and ask how to switch your router to bridge mode.

Note that if your ISP router has multiple Ethernet ports, only one will remain active after ISP router is put to bridge mode. You may need to try all of them to find which one is that, if it is not the first port on router.

It also means, you can only have Main Deco connected to ISP router running in bridge mode. If you have something else connected to ISP router by Ethernet cables, you'll have to reconnect that equipment elsewhere.

You will also have to review how your Deco units are connected to each other, there are restrictions if you run Deco mesh in router mode. 

Finally, I am assuming you already disabled WiFi on ISP router, but if not - you should, otherwise a user may bypass Deco filters by connecting to WiFi network provided by ISP router.

My suggestion would be to switch Deco to router mode first, and then switch ISP router to bridge mode if you need to.