Update:

Finally, all works correctly. I did something wrong and mixed-up two IOT's MAC addresses beleiving then the IP reservation did not work. But it does.
So, when the DECO is set on Access Point, the IP reservation is managed on the router side (here, my Archer AX55) and it works as expected.
Just to highlight that the nodes connecting on the Deco X50's are seen as 'wired' on the router side (Archer AX55) since the root Deco X50 is wired connected on the router.

Hi,

I've just received a pair of DECO X50 I've connected in Access Point to my Archer AX55.

Apparently, everything works perfectly well. JUst be aware I've kept right now the Wifi on Archer. So that I've two distinct Wifi's :
ArcherWifiName : On archer
DecoWifiName : On Deco.

However, I'm in troubled with IP reservation mode for the devices previously on ArcherWifiName and reconfigured to use DecoWifiName (I've read this FAQ but it is not relevant for a DECO connected in Access Point).
 

1. The migrated devices from Archer Wifi to Deco Wifi has got a new IP address and so, the reserved IP address on Archer does not apply.
2. I see the list of the migrated devices on the Archer connected clients but again with an IP address not maching the one that is reserved on Archer.
   Note that the the new IP address is on the same IP network.
3. After the configuration of DECO in Access Point, menus for IP reservation mode are no longer available in DECO App and also, there is no way to configure the DHCP subnet.
   I assume in AP mode, DHCP config is done by the router but then,
   why don't I receive the reserved IP address in DECO Wifi ?
4. I see the device both on the Archer 'Connected Client' list and on the DECO client list with the same IP address (obviously) but not the one that is reserved.

Should I use the same Wifi name on both Archer and Deco Wifi ? Should I deactivate the Wifi on Archer ? 

Help would be higly appreciated.
Thierry from Belgium