@Wabbak 

Both Deco M9 plus and Deco P7 have their own pros and cons; 

For example, Deco P7 supports powerline backhaul, which makes the connection more stable; While the Deco M9 plus has a separate 5GHz-2 WIFI backhaul. And Deco M9 plus is a smart hub.

For more information, you can refer to the faq below or read the user manual.

https://www.tp-link.com/support/faq/2423/

Besides, you can add more than 1 Deco into the home network, about their priorities, please click here.

May it help and have a nice day. 

@Kevin_Z 

Hi all

in addition to @kevin_Z answer I have a question:

I have a set of 4 Deco M9Plus routers. With all the working from home its an excellent stable network.

Now we are building a veranda in the garden, and there is a dead spot. Is it possible to combine a P9 with the M9Plus, while using the same network. The same network is important, because I'm using some software in the house which should be on the exact same network, otherwise it doesn't work.

Thanks,

Fulco

@Fulco 

You will need to have two P9 units to establish powerline link between them.

If you want to use powerline backhaul, considering your current Deco setup it may be better to buy a pair of AV1000 gigabit powerline adapters and a single M9 or M5 for veranda.