Don't know if I'm too late or not, but here's my penny's worth.
As other guys pointed out, creating crossing between Karu and Naresu Plateau requires access to Filia, meaning the can opener is inside the can.
You have few options.
a) Ask an elf to take you there in Uladh. If you can find a volunteer elf, this would be the fastest way.
In detail. Form a party (you form one or he/she form one is not issue). Have the elf to summon his/her mount pet. You hop on it, and get elf to do continental warp. You'll be in Filia. The end.
b) Take a detour into Courcle. Rather than the edge of Karu, go to northern east most of Meiz Prairie, after picking up L rod.
[Video: https://youtu.be/rOHFX40HK8U?t=3m34s]
See vid for specific location. Once you enter Courcle, travel east and you should find terrain with similar "dent" by cliff at eastern end of Cenae Meadows. Use L rod again here to build bridge into Connous. That will take you into Erans Gorge, just west of Filia. Warning: lots of bears near the end of bridge.
Just use map to orient yourself, travel south, then east into Filia.
Once you're there, you might want to pick up the bridge making quest,, since you will get rather useful title and you'll leave your name on the land, for a week :-)
see Cookie-san's post how to proceed with quest.