I had FastestVPN working perfectly on my Dreamlink T3. One day it just suddenly froze connecting to a server and wouldn’t allow to change server or even close the app. The only thing I could do was click logout. Since then I can no longer login on this box. Works everywhere else on my other boxes, computers and phones. I contacted support and they had me install an old version and that gives a Java security error. I have tried other older versions and same problem.
Not clear if you are using the FastVPN app, or just an OpenVPN client. Perhaps, just switch to an OpenVPN client, and use the config files that FastestVPN provides for it?
I have had random issues with fastestvpn app even on iPhone. Sometimes as said previously a mere update of app may make it work. Also try signing out and signing back in. Otherwise uninstall and reinstall.
I’ve been getting ipv6 errors out of nowhere using the app and OVPN apps on Mac, but other devices connect on the same network. Never has ipv6 issues until a week or so ago