Actually I believe attaching a properly configured router/switch to each SMEs internal network might be the only way using "add network". Sadly, since this would mean having to get and configure more than two dozens routers for locations SMEs
Absolutely not. You just need to understand your networking and how SME 'Local Networks' work. SME is perfectly capable of doing this without the need for more equipment which will just confuse things.
Local networks are for when you are trying to route from one local network to another local network via 'somewhere'.
It isn't well documented because there are a huge amount of possibilities and it isn't easy to document them all, though I would agree it could have a basic example. You have two different examples here already.
So you want to make 192.168.1.0 visible to 192.168.2.0 and vice versa
192.168.1.0 <-> 10.0.1.1 <- This could be via a local network, Internet or whatever -> 10.0.2.1 <-> 192.168.2.0
Local network - as per Mikes comment on routing
On 10.0.1.1 we need to tell it where to find the other networks.
Lan 192.168.2.0 << this is the Lan we want to access
Subnet mask 255.255.255.0 << This is the subnet mask of the Lan you want to access
Router 10.0.2.1 (effectively via
this gateway - it
has to know where 192.168.2.0 is)
Lan 192.168.3.0 << this is the Lan we want to access
Subnet mask 255.255.255.0 << This is the subnet mask of the Lan you want to access
Router 10.0.3.1 (effectively via
this gateway - it
has to know where 192.168.3.0 is)
On 10.0.2.1 we need to tell it where to find the other networks.
Lan 192.168.1.0 << this is the Lan we want to access
Subnet mask 255.255.255.0 << This is the subnet mask of the Lan you want to access
Router 10.0.1.1 (effectively via
this gateway - it
has to know where 192.168.1.0 is)
Lan 192.168.3.0 << this is the Lan we want to access
Subnet mask 255.255.255.0 << This is the subnet mask of the Lan you want to access
Router 10.0.3.1 (effectively via
this gateway - it
has to know where 192.168.3.0 is)
On 10.0.3.1 we need to tell it where to find the other networks.
Lan 192.168.1.0 << this is the Lan we want to access
Subnet mask 255.255.255.0 << This is the subnet mask of the Lan you want to access
Router 10.0.1.1 (effectively via
this gateway - it
has to know where 192.168.1.0 is)
Lan 192.168.2.0 << this is the Lan we want to access
Subnet mask 255.255.255.0 << This is the subnet mask of the Lan you want to access
Router 10.0.2.1 (effectively via
this gateway - it
has to know where 192.168.2.0 is)
Access to the AV server - if we do this as a local lan then it is required on all 3 SMEs. It does not have a 'local network' behind it. So we can add it like this:
Lan 10.1.3.50
Subnet mask 255.255.255.255
Router 10.0.3.50
(The AV server may require reverse routes to all 3 local networks.)
However, as Mike said, this is probably not necessary as you can create routes on your Router & Firewall [LAN 10.1.x.x] for this.
So any machines on the 192.168.x.x looking for the AV server query their own gateway, that then queries the main gateway that then tells it where the AV server is.