Ok - first - I would think I'm about the last person who should be offering an answer - being as I am a brand new transmission user. But, I fail to understand how a link to an EIGHT year old issue is applicable today - at least - not without some sort of explanation, eh? I even went to the link, and fail to see how it applies, other than remotely, to the op request.
However - in getting my transmission install started - I've learned a thing or two.
So check out this page:
https://trac.transmissionbt.com/wiki/EditConfigFiles
for these settings:
peer-port: Number (default = 51413)
peer-port-random-high: Number (default = 65535)
peer-port-random-low: Number (default = 1024)
peer-port is where transmission listens for peer requests. The next two "-random-high" and "random-low" can be used to limit the port range for outgoing traffic. I set mine to the very same port I used for peer-port, as I have seen other forum posts saying that the poster got such a setup to work.
E.G.
peer-port: Number (default = 50100)
peer-port-random-high: 50100
peer-port-random-low: 50100
I'm still getting my setup operational, so I won't say the above range restrictions work. But I think they will, based on other forum's posted answers to similar issues.