Back to My Mac issues

Hi,

For the last 2 weeks my iMac has been showing this message:

Back to My Mac may be slow because NAT Port Mapping (NAT-PMP) or Universal Plug and Play (UPnP) is turned off on your router.

I have contacted BT technical department and they say that this has nothing to do with the router but more likely a configuration problem with the iMac ?

All of our other Apple products that are connected to the same iCloud account are working fine.

All advice welcome !