@David-TP 

May I know how you found out Deco X55 does not forward DHCP requests anymore, for example, all the clients start getting IP addresses from X55, instead of the main router; or the clients are no longer getting addresses anymore?

We have replaced the DECO M5 with the Deco X55 in the same setup and simple the Deco X55 doesn't forward DHCP, but if you setup a static IP in each client device by WiFi or cable it works. 

If it is the first case, there is a beta firmware that has added an on/off switch for "Smart DHCP" under access point mode and you could try to install the beta firmware and turn off the DHCP to have a look.

We have been able to reproduce the ERROR and how to make it happen.

1) Install the DECO as ADD to the existent mesh network in AP mode, 1st unit by cable,

2) 2nd by wifi.

3) UPDATE ALL FIRMWARES

4) try to get clients to get IP over the new Deco X55 device. Problem occurs.

remove the old decos from the network and make the X55 the main unit. Voila, the problem occurs.

5) Now change the network to Router Mode.

It Works.

6) Now change it back to AP mode

it works too.