Challenge: Tibet with sea access

Tibet is a region very much confined to inner Asia for most of its history, but given that incarnations of the Tibetan state have ad sea access, what's the latest POD for an independent Tibet with sea access, presumably via the Brahmaputra River/Bay of Bengal?
 
I know about the Tibetan Empire. I'm talking later in history. The Brahmaputra is navigable for much of its length, as I understand it. So, could a Tibet-based, predominantly culturally Tibetan, state have such a port, and, if so, how late and under what conditions?
 
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