@Gerbert1970
The Deco units can connect to each other by wired cable or wireless and they support access point mode and wireless router mode; but the main Deco, no matter what operation mode it is, it need connect to the main router/modem by wired cable to get internet first.
Back to your case, you can add Deco M9 plus as the access point to extend the wifi range, but the one of the Deco need connect to the C5400 by wired cable directly. You can read https://www.tp-link.com/support/faq/1432/
Or you can use the Deco M9 plus as the regular wireless router and configure the C5400 as the access point or repeater.
For the difference between router mode and AP mode, please refer to https://www.tp-link.com/support/faq/2399/
May it help and have a good day.