Hello,
you will need to add rule in firewall via terminal.
SSH to router and edit "/etc/config/firewall" file with vi editor.
Find lines about wan zone:
config zone
option name 'wan'
option network 'wan ppp wan2'
option input 'REJECT'
option output 'ACCEPT'
option forward 'REJECT'
option masq '1'
option mtu_fix '1'
Add following line:
list masq_dest '!10.0.0.0/8'
This should work.