Disable split tunneling with L2TP

I'm not sure I understand what you're asking for. You say you want to turn off split tunneling, but then you say you want to 'set different network routes when the VPN is connected', which is the definition of split tunneling...?

Anyway, there are two places you can control this. On the VPN server you should be able to tell it what networks to send over the tunnel (set it to all). On the client side, in Internet Connect, select the VPN connection and select the Connect -> Options menu. Here there's a checkbox that says something like 'Send all traffic over VPN connection'. Set it however you prefer.

I get an IP like 10.54.226.0/24 when I connect to the VPN. But I also want to route 192.168.1.0/24 and 192.168.2.0/24 to the VPN.

Typically, this is controlled by the VPN server. You don't say what server you're using, but usually you can tell it what subnets to tell the client to send over the tunnel.

> the VPN server does not control the routing table of the client

Not directly, no, but it does tell the client what subnets are available over the VPN on the expectation that the client will adjust its routing table accordingly.

So technically you're correct, but that's not to say the server doesn't have any involvement.