So this is an old thread but I’ve had the same issue and resolvevd it today. I have a Surfboard 6183 modem (not a router and no wifi built in) running Xfinity/Comcast broadband and a 3 unit Google WiFi system. My tablo is hard wired in via cat5.
Previously I could connect to the tablo remotely from outside my house. It would fail from time to time but mostly worked. For the past several months I have not been able to.connect.to my tablo remotely. I had previously setup the port forwarding on my.googke.wifi system but thatndid kit fix it.
Today I turned off UPnP on my.google.wifi stlystem.and suddenly my tablo.would connect remotely again. Hope that helps.