According to
https://wiki.openwrt.org/doc/uci/network , try to add these 2 lines in bold :
config interface '
wan' <-- do not call it
modem option type 'bridge'
option proto 'none'
option auto '1' option ifname 'eth3 eth4 eth5 ptm0'
Option auto mean "start this interface on boot", and
proto should be set to
none, i don't know what protocol is used if it is not set, so better write it.
Anyway i think you removed a lot of things in the config file...
why did you remove eth0 eth1 eth2 from the
option ifname ?
2nd thing, in bridge mode you don't need the modem firewall, so :
/etc/init.d/firewall disable
/etc/init.d/firewall stop/etc/init.d/network restart to apply all the changes.