Provider's box : 192.168.1.1
OPNsense WAN IP is 192.168.1.2
OPNsense LAN IP is 10.0.0.1
and I activated DHCP on LAN
- are you sure there is only ONE active DHCP server in your network....?
192.168.1.1 is (I guess) an ISP router/modem, that has probably got an active DHCP server running.