You will probably need to retain an attorney to resolve this matter for you. The so-called Open Range Law is actually a collection of separate statutes, first enacted in the 1870's and amended many times since. Various judicial decisions have modified the statutes even further. The law is not as coherent and self-explanatory as it might originally have seemed.
Your basic premise is correct in that you are not liable for damages caused by (or to) livestock damaged while on a highway going _through_ open range land. Note that the highway has to actually pass through, not alongside, open range land. Being open range on one side and fenced on the other creates a whole different scenario (!). Also, it could be relevant to determine how the cows got onto the highway -- if they passed through somebody's fences (even if not your fences) which were negligently left open or down, that landowner could be liable for the damages.
Long story make somewhat shorter, in dealing with an insurance company you will almost always need to retain an attorney. The coverage obligations will be determined by an examination of the terms and conditions of the insurance policy, which is simply not something which you can do on your own. The overall fight with the insurance company -- if you really want to win -- is also not something which you can do on your own. Many attorneys will offer a free or low-cost initial consultation. If you can locate one, an attorney who is well-versed in farming or livestock issues will probably be of most benefit to you.
That's not a question of law, but rather a question of what your policy covers. If they're worth enough to justify the cost, take your policy in to an attorney for review of your coverage.