The problem with Norway is its ether flat or vertical... you just don't get the rolling landscape you would imagine - well you do - but just not in "fjordland".
Roads and tracks i the fjord areas are squeezed in the valley bottoms with a nice new road and the old road now a gravel track next to it (often the much more spectacular option). The plateaus on the"the tops" are mostly devoid of cycling opportunities due to the very nature of the terrain (no easy way to get on the top of most plateaus due to the vertical sides of the Fjords) and lack of historic use. Also Norway has very little in the way of commercial forestry, so using fire roads is generally not an option... as they don't exist!.
Having said that, as mentioned, the Hardangervidda Plateau National Park has a good fixed route you can cycle across (complete with mountain hut accommodation). But it also has loads of walking tracks that you can cycle on, but are not dedicated for cycling . Outdoor access is the same for foot or bike - except bikes can be banned from certain sections of mountains.
I worked up in Trondheim (Stjørdal) for a a few years and found it to be the best place for cycling in Norway (and home to Norway's only outdoor Velodrome) Yes its less "dramatic" but still interesting riding and heading inland to Sweden is very pleasant. Its like a more interesting version of Mid Wales but much greener but devoid of "gravel" in the america sense but loads of x-country ski trails, esp. in Sweden . Could be working back in Stjørdal this summer
Also worked in Bergen for many years and found it also a great place for cycling, as long as you except the limited routes due to the geography in "fjordland" for off-road riding. The "Atlantic Road" to the south is crap btw.
I now work in Stavanger and its quite different here. We have a long strip of flat, farm lands heading south for about 150km, so you have that but just 5km inland and its all mountains and byways - even the "main" roads will only have a minimal amount of traffic when you head inland.
I suggest you have a look at the satellite images of the areas you would like to be riding as see if there is actually anything on the ground - its what we do and the only real way of knowing if the track goes to a vertical cliff or can be used as a route to go places. Its a big, wild place with little in the way of tracks out in the wild due to lack of human traffic.
There is a reason for the lack of numerous routes on the web that are all off-road in Norway