Routing non-routable addresses

Sorry for the dumb question: how do I debug ip-forwarding?

My setup is as follows:

• I have a Linux SoC connected via serial line to a Linux laptop

• over that serial line I have a working PPP

  sudo pppd updetatch connect 'chat -f chatscript' noauth nocrtscts local 192.168.8.1:192.168.8.2 /dev/ttyUSB0 115200

• the laptop on the other end of the serial line has regular net connection (192.168.7.xxx)

• I added a default route on SoC pointing to the remote PPP

  ip route add default via 192.168.8.1

• I enabled ip_forward on laptop

  echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward

This is not enough to have a working network though.

In order to reach the Internet I should enable NAT on laptop, but this is not my primary concern.

I checked ip-tables on laptop and they all show

`Chain [INPUT/FORWARD/PUTPUT] (policy ACCEPT 0 packets, 0 bytes)`

My real problem is I would need to route form SoC to some other machine sitting om my regular LAN (192.168.7.0/24).

I assume I need to add some sort of static route as addresses in 182.168.x.y are not routable by default.

I am stuck here as I didn't find a way to ip route add ... to make it working.

I am able to ping from SoC to Laptop and vice-versa, but nothing beyond that.

What should I add?