From the man pages
-S, --local, --server=[//[domain/]][[#][@|[#]]
The optional string after the @ character tells dnsmasq how to set the source of the queries to this nameserver. It can either be an ip-address, an interface name or both. Forcing queries to an interface is not implemented on all platforms supported by dnsmasq.
on my dhcp file, I have added the below lines to the dns section.
option server '188.8.131.52@tun0' option server '184.108.40.206@tun0'
It does not seem to be working. Is the feature supported on LEDE?