September is autumn over here and the weather can be cold and very wet. The trout can often be in spawning mode the further north you go. But with that said, the fishing can still be good.
This year hasn't been the warmest, with large amounts of rain. The water temperatures can therefore be somewhat reduced.
In Evje you have Surtveitsfossen, which is the waterfall that runs out the sluices in the southern end of Byglandsfjord. From the sluices and 1km downstream is very good fishing. Here you've got the biggest chance of catching an inland salmon called, bleka. I believe Byglandsfjord is one of only 3-4 waterways that has this fish. Further north so have you very good fishing around Hovden.
In Fyresdal you've got some very good whitefish lakes, with fish upto 5-6lbs a regular occurance. I fished here last year, but only caught small trout. I was lucky enough to get lots of local knowledge and loan a boat to fish fyresvatn, but no whitefish unfortunately.
Hardangervidda is very good. West is best but I've fished the east in september with varied results. One tip, if you find a lake with rising fish, stay there!
Take a bellyboat, as some lakes are very hard to fish from the bank. Be careful, remember a life jacket as your often a long way from help!
Good luck, and enjoy Norway