I just learnt this yesterday. Firewall settings for two site-to-site OpenWrt routers - #7 by vgaetera
Goto Advance of your WAN zone with masquerading enable. Note the '!' before the destination subnet - it means traffic going to that subnet will not masquerade.
The uci command for my router of that option is:
uci add_list firewall.@zone[1].masq_dest="!192.168.104.0/24"