Try this :
From your SME, get your ISP newsserver IP address (with dig, ping, etc.) and use this IP on your clients computers.
I don't known if this will resolve your problem but we have a similar problem in France with an ISP that checks your own IP and, depending to it, doesn't resolve DNS requests with the same answer.
If your own IP is one of the IP the ISP provides, it resolves with the good IP adresses of its own servers.
When you are behind your SME box, your own IP is the
local one, so the ISP doesn't recognise it and it sends a false IP to a DNS request for its servers.
(not simple for me to explain that in english, sorry

)