Hi, finally was a problem with the firmware file that i was using. (i dont know why)
I tried with an older one from a .de mirror repository and it worked doing it using tftp in ip 192.168.0.1 (default ip for this router is 192.168.5.1).
Then i configued it as modem router for bt adsl (uk), i put here the config for anyone wanting to configure a modem router lantiq to work with adsl because there is not a lot of documentation about this topic.
Kind regards.
NETGEAR DM200 LEDE MODEM (BT-UK) CONFIG
cat /etc/config/network
config interface 'loopback'
option ifname 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'xxxx:xxxx:xxxx::/48'
config atm-bridge 'atm'
option vpi '0'
option vci '38'
option encaps 'llc'
option payload 'bridged'
config dsl 'dsl'
option annex 'a'
option xfer_mode 'atm'
option line_mode 'adsl'
config interface 'lan'
option type 'bridge'
option ifname 'eth0'
option proto 'static'
option ipaddr '192.168.2.10'
option netmask '255.255.255.0'
option dns '8.8.8.8'
option ip6assign '60'
config device 'lan_dev'
option name 'eth0'
option macaddr 'xx:xx:xx:xx:xx:xx'
config interface 'wan'
option ifname 'dsl0' #dsl0 original
option proto 'pppoe'
option username 'usebtusername'
option password 'btpassworddontmatter'
option ipv6 'auto'
config device 'wan_dev'
option name 'dsl0' #dsl0 original
option macaddr 'xx:xx:xx:xx:xx:xx'
config interface 'wan6'
option ifname '@wan' #@wan original
option proto 'dhcpv6'