OK, I just talked to Zyxel support on the phone.
From what I understood connecting to to CISCO 4050 via IPSec is possible.
[Correct me if I am wrong] Connecting to another site via L2TP is not possible because L2TP as a protocol is not meant for router-to-router connection - it is only meant for individual computers to connect TO the router. If I want to connect to this device:
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www.amazon.com/8-port-Fa ··· n+routerit needs to support IPSec for connection INTO it.