You don't need to be high lvl fishing to break a mithran rod, you can do it with lvl 0 fishing. I did it and have goten friends to do it b fishing in Qufim on the Behemoths Dominion side across from Delkfutts Tower. I don't remember the exact name of the bait but it's sliced bluetail I think, I know its sliced something. You shouls be able to break the rod within 20 minutes if you haven't already done so. Some people make the mistake of trying to not catch the fish, you need to catch the fish for it to break, it'll give oyu a message of you have doubts, or you know you cant catch this fish, if you wear it down and attempt to bring it in it usually breaks.