Huge Latency Spikes Every Few Seconds

Hello, since September I have had an issue of huge latency/ping spikes every few seconds. My internet speed is just fine, but these spikes in ping make it borderline impossible to play online games. 

This occurs on all devices, regardless of whether they're using WiFi or Ethernet, these spikes do not appear when pinging my default gateway (they are not related to any devices' connection to the router itself). No engineers I have had out have managed to fix the issue. I have been looking over the internet for a solution for a while now and haven't found one. I am not sure what else to do at this point.