The airlines will not restrict it coming on board unless it contains oxygen or oxygen generator. I believe there has been some fairly recent changes with the FAA concerning personal oxygen generators, but I honestly don't remember now what the changes were.
Airlines to their best to accommodate those with special needs. I do not know how large your rolling duffel bag is, but most airlines do post the maximum overhead bin stowage measurements on their website (somewhere). Preferably you would be able to stow it under the seat in front of you (for your benefit).
My best advise would be to contact the airline, advise them that you will need it to go on board with you.
If you request, or need a bulkhead seat, you will lose the stowage space that you would have if you were sitting in a row with seats in front of you. If you have batteries for it, that is another thing to verify as afar as if it can go on board with you. The FAA has strict guidelines as to what may ride in cargo versus what may go into the cabin with passengers. (which is why the wheelchairs with the large lithium battery must go into the cargo hold).
Make sure the airline knows that you will need to pre-board before the main steam of customers, and that if your cough assist device is not where you, or someone with you can get if for you, that the crew will need to be advised of the situation, and it can be brought to you if needed (if it is stowed away in a coat closet for instance) If you pre-board though, then you can make sure you are able to find space to store it within reach.
If the duffel bag is small enough to fit in an overhead bin, label it on with a clearly marked luggage tag that it is not to be moved without your knowledge. Be bold about marking the tag, and make sure the tag is visible.
Keep in mind that the stowage space under an aisle seat is narrower by at least a third of space under a middle seat or window seat. I'm assuming that your cough assist device is about 12" square, (give or take) which might not fit easily under an aisle seat, but should go just fine under a middle or window one.... :smile: