Right, found the problem. Well one of them…
My issue was lack of RAM. I only have 512MB on the ZEROSHELL box. Although it’s a server it’s a very old one.
Now that I “understand” a bit more about zeroshell I think I have the proper questions to ask. Here goes:

1st – Should zeroshell “management interface” be on the fiber network (were the gateway is) or the office network?
2nd – To enable office devices access the fiber network do I need to enable NAT on the fiber or the office network?
3rd – Without using Zeroshell as the DHCP server how can I “force” all clients to use zeroshell as a gateway or proxy?
Many thanks in advance ! In the meantime I’m configuring another box for my tests.