route from internal network to external ip

Hello,

In a home situation I have 3 local subnets that are natted(prerouted?) through a server on a fiber network(100mbps) the lan is a gigabit network and now I've noticed that backups to the server(external IP due to use of laptops) go at a max rate of 100mbps.
So I'd like to add a route to my external IP on the server so it'll know not to use the limitations of my external interface.

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.2.0     *               255.255.255.0   U     0      0        0 eth0
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
192.168.0.0     *               255.255.255.0   U     0      0        0 eth0
88.159.x.0    *               255.255.254.0   U     0      0        0 eth1
default         x-x-ftth.on 0.0.0.0         UG    0      0        0 eth1

How and what do I add?

Do you means you want to setup routing for multiple uplinks/providers?

no all I'd like to do is add a route so nodes on my local network will go straight to localhost when they access the external IP from the lan so they are not limited by the "slow" uplink to the gateway on the 88.159.x.x external network.

A simple way, each client add a route to handle 88.159.x.x traffic and gateway is your server. And in your server, a route send back 88.159.x.x traffic to your LAN.

problem is that I can't add static routes to all the nodes in the lan. Just in the gateway... but 88.159.x.x is an actual ip not a subnet. It's just the external IP of the gateway server

Set following route's device to loopback interface, eth1 should be lo.
88.159.x.0 * 255.255.254.0 U 0 0 0 eth1