Marco,
PE1CHL assisted me with this some time ago, the reasoning is outlined here:
http://wiki.ampr.org/wiki/Setting_up_a_gateway_on_OpenWRT
https://en.wikipedia.org/wiki/Virtual_routing_and_forwarding
You need to tell your interfaces and IPs to use specific route tables.
This is done on Linux using the command: ip rule
# OPTIONAL# ip rule add from <your AMPRLAN> to 192.168.0.0/24 table main
priority 22
ip rule add to <your AMPRLAN> table main priority 44
ip rule add dev tunl0 table 44 priority 45
ip rule add dev br-amprnet table 44 priority 46
ip rule add from <your AMPRLAN> table 44 priority 47
73,
- Lynwood
KB3VWG