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If you set the default route to your cellular interface (ppp 1) then it will route all incoming traffic from the Ethernet interface(s) out that interface. Here is the user manual entry for default routes:
As far as your IPsec question, if your default route is set correctly and the cellular interface is the default interface for WAN connectivity then it should route that traffic correctly. If you need additional assistance after setting the default route, please open a case at firstname.lastname@example.org and please provide a copy of your debug.txt file:
Digi Technical Support Engineer