What kind of modem are you using? Or is that Linksys a combo modem/router/access point?
When you go back to the Tablo\Setting\Tablo Connect are you able to click on the Re-Test Port mapping button. Do you get the same error?
Iām wondering if there may be ISP port blocking in play. You can run a test by going to www.portchecktool.com or similar site and entering the external ports. That would at least confirm or rule out this as the issue.