Feedback:
ESXi 6.5 did not work same problem.
Some progress:
It seems that there is some problem with DHCP, the DSM 6 is not requesting/getting ip correctly.
I tried force set ip dirict to MAC:
arp -s 192.168.1.236 00-0C-29-BB-AF-89
THIS WORKED...not as expected, but it seems that it "fired up" the DHCP client in the DSM and now it got an IP.
Not the ip I tried to set, but another ip from the DHCP server. I can now see it with Synology Assistant.
I will investigate more when I have time, but for now I can access the DSM.