So i had the same issue. after investigating, the 406 error started happening after Spectrum( my ISP) issued me new IP address. When i forced a new IP change to a new IP address the issue went away. I called Ring customer service over the phone and gave them the problematic IP address. For some reason they are blacklisting IP addresses that have given them problems in the past and keep them blacklisted. So when those problematic IP addresses get reissued to innocent ring users… you have this problem. After a useless hour on the phone I solved my own problem. But Ring support was useless. I can appreciate their focus on security but evetually they will run out of IP addresses to blacklist and will end up with many more furious ring users if they don’t take a different approach to security.
So to the workaround - every home router has the ability to clone MAC address. In my case on my netgear router i went to the internet port setting and told it to use my PC’s MAC address instead of the default MAC address. When your cable modem or ISP sees a new MAC address, it will issue a new IP address. This is a whole lot easier than calling your ISP and trying to explain why you need a new IP address. You can probably switch back to default MAC address on your router at a later date since IP leases are always changing over a predefined period of time setup by your ISP.