@DutchIT 

It should be a DSL service since it uses a telephone cable. If that's the case, this is the modem/router from your ISP and you can't bypass it. Moreover you shouldn't have RJ-11 (telephone cable) connector on your M9+ devices.

Now, what you can do is to ask your ISP, if the modem/router can be configured in bridge mode. If it can, then you can use your main M9+ device in router mode to avoid double NAT in your network.

Of course you'll have to disable your old wi-fi network, so it won't interfere with the DECO system wireless signal.