Q: Back to my mac doesn't work for remote network

I uses TM (timemachine) as my router, which is connected to comcast cable modem.

I can connect imac from macbook via 'back to my mac' when both are in the same network, but I can't if macbook is on different network (e.g. at starbucks).

However, I can still logging in TM and see all the files even from remote network.

Is there any special configuration required for either TM (Airport) or IMAC to access remotely using Back to my mac?