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I "forgot" my Internet in settings, turned off the VPN and restarted the firestick. Connected to WiFi again and then connected to VPN and it worked. I've not had an issue since.
I've had the exact same before as written above on 4k firestick.I’ve had this before and found the problem was that the VPN was auto connecting and causing some sort of clash. Before you do anything else, go into your VPN app and disconnect, switch off auto connect if it’s enabled. Now go ahead and reconnect your WiFi network, the VPN should no longer be clashing. Once the connection is back, now try connecting to the VPN manually.
I "forgot" my Internet in settings, turned off the VPN and restarted the firestick. Connected to WiFi again and then connected to VPN and it worked. I've not had an issue since.