Use iproute2

[ymolinet]

Default SNAT rule (LAN to WAN) do not appear in SNAT rules (Web GUI) ?

[christian]

My first idea is that packets come back by the default (WAN3) interface instead of the original WAN.

+1, I've exactly same feeling. You perhaps need to replace source address (oh no ! another SNAT related debate    )

[ymolinet]

Default SNAT rule doesn't appear in Web GUI ? how to change priority ?

[christian]

I can't help, I don't understand your question neither what you mean with "priority" 

[ymolinet]

it seems that the default SNAT rules (LAN to WAN with SNAT on default WAN interface) do not appear in the SNAT rules list ? you confrm ?
my snat rule for a specific server must be "read" before the default (order instead of priority ?) so if default rule do not appear, is custom rule read before ?