@kAndy83
Hi, yes.
You might need to put both M4s into access point mode.
There are detailed instructions:
1. Unplug one M4 from the Wi-Fi modem.
2. Hold the reset pinhole for 1s to reset it first.
3. Refer to this link and add this Deco into the first M4 mesh.
4. Go to More>advanced>operation mode and change to access point.
As for the modem, you might need to turn off the Wi-Fi on the modem if you do not want to have two separate wireless networks.
(or you could change the Wi-Fi name of the Deco to be the same as the Wi-Fi modem but please note one thing even though they share the same name, there would still be some delay when switching one to another. )
How to change the Wi-Fi name of Deco: go to Deco APP>More>Wi-Fi>network name(SSID)