I have this problem too....However, depending on your router access, you may be able to fix it.
Thanks to xfinity putting port access into their app (which requires account details beyond the router login for me, boo), I cannot fix this problem for myself....but, if you can log into your router, you CAN probably solve your problem. Its pretty straight forward, and the site SLSK sends you to breaks down how to do it pretty simply--if you have even the most basic of computer understanding, I'm guessing you'll be able to figure it out.
If you click the blue "click here" link, it takes you to the slsk site, which suggests that you go to a DIFFERENT site that is super user friendly, with details on how to open ports for a whole plethora of breeds of modems and routers.
If you aren't sure what you have, take a picture of the bottom/back sticker (the thing with all the identifying features like the manufacturer and model), as well as your internet provider (not always necessary, but it will cut down on the back and forth) and post it here or email it to me, and I can give you the link that specifically applies to what you have.
Hell, if you catch me on the right day, I can even walk you through anything that is still confusing. Just have to plan for it. Feel free to ask. If I have a bit of free time that works with your schedule, I got yew.