I agree that the tethering driver is a likely suspect.
If you can, eliminate WiFi as a suspect by plugging into the Surf SOHO via ethernet. I know Apple is removing ports from all their devices, but USB to ethernet adapters are tried and true and cheap.
Also, when you next do this, try watching the live bandwidth report on the router. Its in Status -> Real Time Bandwidth. Don’t know what to look for specifically but it may show a pattern.
Finally, there is a bandwidth speed profile for each WAN connection. Try adjusting that to be somewhere in the range of reality. On the Dashboard (main router screen) click the gray Details button next to your WAN connection and look for Upload bandwidth and Download bandwidth.