@hubertk 

ZTE Automatic mode (no bridge, no anything else just automatic) One M4 connected with the cable to this router, second one wifi on the other floor. The third one still in the box.

I have deco on "Router wifi" mode. Is it ok?--yes, it will work fine.

 

I have tried to switch ZTE on bridge mode but in this situation DECO blinks red - no internet at all. Why is so?

I think you could unplug M4 first and connect a laptop to the ZTE modem. If the PC worked, Deco M4 should work as well. If the PC did not work automatically, you might need additional internet set-up on the Deco, like a pair of username and password provided by ISP.

Best regards.