Ok, an instant connection with this kind of "ethernet over ADSL" internet access can be done with this command in the router:
br2684ctl -b -c 0 -e 0 -p 1 -a 0.8.32
But I've found the way to save the configuration permanently for OpenWRT v10.03 in order to be loaded on boot by /etc/init.d/br2684ctl : editing the /etc/config/network file in the router.
I've left the WAN interface section as this:
config 'interface' 'wan'
# option 'defaultroute' '0'
# option 'peerdns' '0'
option 'proto' 'dhcp'
# option 'ifname' 'eth1'
option 'ifname' 'nas0'
And added a new section for ATM bridge:
config 'atm-bridge'
option 'atmdev' '0' # ATM interface nr.
option 'unit' '0' # interface nas<num>
option 'vpi' '8' # Virtual Path Identifier
option 'vci' '32' # Virtual Channel Identifier
option 'encaps' 'llc' # encapsulation (llc=LLC, vc=VC Mux)
option 'payload' 'bridged' # payload type (routed, bridged)
# option 'qos' # I don't know how is specified
I don't know if this can be done with LuCI web interface.
I edited easily that file with the "nano" editor:
opkg update; opkg install nano