I am also having this issue. Default gateway not available, tried the listed fixes with no effect.
HP Spectre X360 - 13t-Ac000 CTO
Running windows 10
A while back I found a temporary solution by changing network adapter drivers (the problem occurred less often) after applying this fix from Paul.
https://h30434.www3.hp.com/t5/Notebook-Wireless-and-Networking/Can-t-find-ethernet-LAN-driver-for-hp...
I recently updated drivers thinking the issue would get even better, but was wrong. I began losing connectivity every 20-60 minutes. Rolled back drivers to the ones suggested in the above referenced post but the issue is still happening much more frequently than it used to with the same drivers.
If anyone has a solution I would very much appreciate it. Thanks!